Question: Elizabeth at 90: A Family Portrait is narrated by the longest-serving heir apparent in what?

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Norwegian royal family: The Norwegian Royal Family is the family of the Norwegian monarch.  In Norway there is a distinction between the Royal House and the Royal Family.  The Royal House includes only the monarch and their spouse, the heir apparent and their spouse, and the heir apparent's eldest child.  The remaining Royal Family includes also all other children, grandchildren and siblings of the monarch, along with their spouses and widows or widowers.

Charles, Prince of Wales: Charles, Prince of Wales (Charles Philip Arthur George; born 14 November 1948) is the eldest child and heir apparent of Queen Elizabeth II.  Known alternatively in Cornwall as Duke of Cornwall and in Scotland as Duke of Rothesay, he is the longest-serving heir apparent in British history, having held the position since 1952.  He is also the oldest person to be next in line to the throne since Sophia of Hanover (the heir presumptive to Queen Anne), who died in 1714 at the age of 83.

Prince of Beira: Prince of Beira (Portuguese: "Príncipe da Beira" ) is a title traditionally granted to the heir apparent of the heir apparent to the throne of Portugal.  The title's original use that it be granted on the eldest daughter of the reigning monarch of Portugal.  Tied with the title of Prince of Beira, is Duke of Barcelos, as heir to the Duke of Braganza and Prince of Brazil ("later" Prince Royal of Portugal).  The current Prince of Beira is Prince Afonso, the eldest son of Duarte Pio, Duke of Braganza.

Line of succession to the Bhutanese throne: The line of succession to the throne of Bhutan is based on the constitution of Bhutan.  Currently the line of succession is according to  male-preference cognatic primogeniture with males preceding females who are in the same degree of kinship.  If the heir apparent has reached the age of majority of 21, the monarch would step down at age 65.  If the heir apparent and the nearest people in the line of succession are deemed unsuitable, it is up to the monarch to decide who will be the next heir.  If the monarch violates the constitution, he or she must abdicate.

Qian Hongzun: Qian Hongzun (錢弘僔) (925－June 7, 940), formally Heir Apparent Xiaoxian (孝獻世子, "the filial and wise heir apparent"), was an heir apparent to the throne of the Chinese Five Dynasties and Ten Kingdoms Period state Wuyue during most of the reign of his father Qian Yuanguan (né Qian Chuanguan, King Wenmu), but did not inherit the throne on account of his predeceasing his father.

Elizabeth at 90: A Family Portrait: Elizabeth at 90: A Family Portrait is a 2016 television documentary film made to commemorate the 90th birthday of Queen Elizabeth II.  It was produced by the BBC and directed by John Bridcut, and narrated by Charles, Prince of Wales.  Aside from archive footage, the programme featured extensive footage shot by the Queen, Prince Philip, Duke of Edinburgh, Princess Margaret, King George VI and Queen Elizabeth the Queen Mother.  The footage had never been publicly shown and came from the Queen's private archive of homemade films.  The footage shown was interspersed with members of the British royal family commenting on the films.  The film featured contributions from the Queen, the Prince of Wales, Anne, Princess Royal, Prince William, Duke of Cambridge and Prince Harry, Lady Sarah Chatto, Prince Edward, Duke of Kent and Princess Alexandra, The Honourable Lady Ogilvy.

List of heirs to the Prussian throne: This is a list of those people who were heir apparent or heir presumptive to the Kingdom of Prussia from its foundation in 1701 to the end of the monarchy in 1918.  From 18 January 1871 the Crown Prince of Prussia was also heir apparent to the German Empire.  Those heirs who succeeded are shown in bold.

Penlop of Trongsa: Penlop of Trongsa (Dzongkha: ཀྲོང་གསར་དཔོན་སློབ་; Wylie: "Krong-gsar dpon-slob"), also called Chhoetse Penlop (Dzongkha: ཆོས་རྩེ་དཔོན་སློབ་; Wylie: "Chos-rtse dpon-slob"; also spelled "Chötse"), is a Dzongkha title meaning "Governor of the Province of Trongsa (Chhoetse)".  It is generally given to the heir apparent of the Kingdom of Bhutan.  The most recent holder of the title was King Jigme Khesar Namgyel Wangchuck, who was then a prince (Dzongkha: "dasho", "gyalsey").  Although the current heir presumptive is Prince Jigyel Ugyen Wangchuck, the title is reserved for the officially designated heir apparent, which is subject to change by the reigning king.  Also, the reigning Druk Gyalpo may retain the office or award it to another person after coronation.  The proper reference style is His Royal Highness Trongsa (Chhoetse) Penlop.

Crown Prince of Thailand: The Crown Prince of Thailand (or Siam; Thai: สยามมกุฎราชกุมาร ; rtgs: Sayammakutratchakuman ) is a title held by the heir apparent to the Thai throne.  First created by King Chulalongkorn (Rama V) in 1886, for his son Prince Maha Vajirunhis, the king's eldest son by a royal wife Queen Savang Vadhana.  Prior to this, the Siamese throne did not have a law or formal system regulating the royal succession.  In 1688 King Petracha of Ayutthaya created the title of Front Palace, which by the Rattanakosin period had become the main title granted to the heir presumptive to the throne.  However few Front Palaces have succeeded to the throne this way, with the exception of King Buddha Loetla Nabhalai (Rama II) in 1809.  After the Front Palace crisis in 1875, the title of Front Palace was abolished and replaced with the title of Crown Prince, who became heir apparent to the throne.

Duke of Rothesay: Duke of Rothesay (Scottish Gaelic: "Diùc Baile Bhòid" , Scots: "Duik o Rothesay" ) is a dynastic title of the heir apparent to the British throne, currently Prince Charles.  It was a title of the heir apparent to the throne of the Kingdom of Scotland before 1707, of the Kingdom of Great Britain from 1707 to 1801, and now of the United Kingdom of Great Britain and Northern Ireland.  It is the title mandated for use by the heir apparent when in Scotland, in preference to the titles "Duke of Cornwall" (which also belongs to the eldest living son of the monarch, when and only when he is also heir apparent, by right) and "Prince of Wales" (traditionally granted to the heir apparent), which are used in the rest of the United Kingdom and overseas.  The Duke of Rothesay also holds other Scottish titles, including those of Earl of Carrick, Baron of Renfrew, Lord of the Isles and Prince and Great Steward of Scotland.  The title is named after Rothesay on the Isle of Bute, Argyll and Bute, but is not associated with any legal entity or landed property, unlike the Duchy of Cornwall.

Supporting paragraphs: Elizabeth at 90: A Family Portrait Charles, Prince of Wales
Answer: British history