Question: The Duchess of Cornwall uses her title instead of what other title, the title held by the first wife of Prince Charles?

Context paragraphs:
Duchess of York: Duchess of York is the principal courtesy title held by the wife of the Duke of York.  The title is gained with marriage alone and is forfeited upon divorce.  Three of the eleven Dukes of York either did not marry or had already assumed the throne prior to marriage, whilst two of the dukes married twice, therefore there have been only ten Duchesses of York.  It is said that the positions of Duke and Duchess of York are charmed, because the title is created every time, or these women become queen consorts.

Duchess of Rothesay: Duchess of Rothesay is a Scottish courtesy title.  It is held by the wife of the Duke of Rothesay since the first Duke in 1398.  Due to the mortality rate and the fact that very few Dukes of Rothesay were of majority or married prior to ascending the throne (if that), there have in fact been only eight Duchesses of Rothesay.  A separate Scottish throne has not existed "de facto" since 1603 when James VI of Scotland acceded to the throne of England when the House of Tudor died out, creating a personal union.  The Act of Union of 1707 united de jure the separate kingdoms and thrones into the Kingdom of Great Britain.  Ever since 1603 the title of the Duchess of Rothesay is held by the Princess of Wales, who was also the Duchess of Cornwall; the title of Duchess of Rothesay is for her use when in Scotland.

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.

Princess of Wales: Princess of Wales (Welsh: "Tywysoges Cymru" ) is a British courtesy title held by the wife of the Prince of Wales, who is, since the 14th century, the heir apparent of the English or British monarch.  The first acknowledged title holder was Eleanor de Montfort, wife of Llywelyn ap Gruffudd.  It has subsequently been used by wives of post-conquest princes of Wales.

Louise de Kérouaille, Duchess of Portsmouth: Louise Renée de Penancoët de Kérouaille, Duchess of Portsmouth (September 1649 – 14 November 1734) was a mistress of Charles II of England.  Through her son by Charles II, Charles Lennox, 1st Duke of Richmond, she is ancestress of both wives of Prince Charles: Diana, Princess of Wales, and Camilla, Duchess of Cornwall.

Duchess of Beaufort: Duchess of Beaufort is a title held by the wife of the Duke of Beaufort in the Peerage of England.  In 1657 Henry Somerset, 3rd Marquess of Worcester married Mary Capell and in 1682 the dukedom was created by Charles II, making Henry the first Duke and Mary the first Duchess of Beaufort.

Camilla, Duchess of Cornwall: Camilla, Duchess of Cornwall, ("née" Camilla Rosemary Shand, previously Parker Bowles; born 17 July 1947), is the second wife of Charles, Prince of Wales, the eldest child and heir apparent of Queen Elizabeth II.  Instead of using the title of Princess of Wales, she uses the title Duchess of Cornwall, her husband's secondary designation.  In Scotland, she is known as the Duchess of Rothesay.  In the event of Charles acceding to the throne, Camilla would become a queen consort; however, an official statement issued by Clarence House on the day of her marriage to Charles announced that she would be known as the Princess Consort.

Duchess of Cambridge: Duchess of Cambridge is the principal courtesy title held by the wife of the Duke of Cambridge.  The title is legally gained upon marriage and is forfeited upon divorce.  Five of the eight Dukes of Cambridge did not marry or morganate, and thus there are only three Duchesses of Cambridge including the current Duchess, Catherine, wife of Prince William, Duke of Cambridge.

Duchess of Cornwall: The Duchess of Cornwall is the title held by the wife of the Duke of Cornwall.  Duke of Cornwall is a non-hereditary peerage held by the British Sovereign's eldest son and heir.

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.

Supporting paragraphs: Camilla, Duchess of Cornwall Princess of Wales
Answer: Princess of Wales