Princess Elisabeth of Denmark, {'1': ", '2': ", '3': 'RE', '4': "} ("Elisabeth Caroline-Mathilde Alexandrine Helena Olga Thyra Feodora Estrid Margrethe Désirée"; born 8 May 1935 in Copenhagen) is the daughter of Hereditary Prince Knud and Hereditary Princess Caroline-Mathilde of Denmark, and is a first cousin of the present Danish monarch, Queen Margrethe II.
The Princess Caroline-Mathilde Alps (Danish: "Prinsesse Caroline Mathilde Alper" ) are a mountain range system in the Holm Land Peninsula, King Frederick VIII Land, northeastern Greenland.
 Administratively this range is part of the Northeast Greenland National Park zone.
King Christian X Land (Danish: "Kong Christian X Land" ) is an area of northeastern Greenland.
Prince Axel Christian Georg of Denmark, {'1': ", '2': ", '3': 'RE, SKmd', '4': "} (Danish: "Prins Axel Christian Georg til Danmark"; 12 August 1888 in Copenhagen – 14 July 1964 in Copenhagen) was a Danish prince and a grandson of Christian IX of Denmark On his father's side, he was a first cousin of Christian X of Denmark, Haakon VII of Norway, Constantine I of Greece, George V of the United Kingdom, Nicholas II of Russia, Maud of Wales and Ernest Augustus III, Duke of Brunswick and on his mother's side of Henri, Count of Paris (1908–1999), Orleanist pretender to the French throne.
 Prince Axel was a popular patron of sports.
 He was a prominent International Olympic Committee member and activist and also a business executive.
 In 1963, Prince Axel became the first honorary member of the IOC in history.
 He was an officer in the Royal Danish Navy.
Prince Harald of Denmark ("Harald Christian Frederik"; 8 October 1876 - 30 March 1949) was a member of the Danish Royal Family.
 He was the third son of Frederick VIII of Denmark and his wife, Lovisa of Sweden, and thus brother to Christian X of Denmark.
Knud, Hereditary Prince of Denmark (Knud Christian Frederik Michael; 27 July 1900 – 14 June 1976), was the younger son and child of Christian X and Alexandrine of Mecklenburg-Schwerin.
The Biddulph Baronetcy, of Westcombe in the County of Kent, is a title in the Baronetage of England.
 It was created on 2 November 1664 for Theophilus Biddulph, of Westcombe Park, Greenwich, Kent, Member of Parliament for the City of London and Lichfield.
 His son, Michael, the second Baronet, also represented Lichfield in the House of Commons.
 This line of the family failed on the death of the second Baronet's son, Theophilus, the third Baronet, in 1743.
 The late Baronet was succeeded by his first cousin once removed and namesake, Theophilus, the fourth Baronet.
 He was the son of Edward Biddulph, elder son of Simon Biddulph, younger son of the first Baronet.
 His grandson, Theophilus, the sixth Baronet, fought in the Battle of Waterloo and served as High Sheriff of Warwickshire in 1849.
 He was succeeded by his son, Theophilus, the seventh Baronet.
 He was a Deputy Lieutenant and Justice of the Peace for Warwickshire.
 On the death of his son, Theophilus, the eighth Baronet, in 1948, the line of Edward, elder son of Simon Biddulph, younger son of the first Baronet, failed.
 The late Baronet was succeeded by his fourth cousin once removed, Francis, the ninth Baronet.
 He was the great-great-grandson of Walter Biddulph, younger son of Simon Biddulph.
 As of 2008 the title is held by his grandson, Jack, the eleventh Baronet, who succeeded his father in 1986.
 The three most recent baronets have resided in Australia.
King Christian X's Liberty Medal (Danish: "Kong Christian den Tiendes frihedsmedaille" ) was a commemorative decoration awarded by King Christian X for special services to Denmark during World War II.
Princess Eleonore-Christine of Schaumburg-Lippe (Eleonore-Christine Eugenie Benita Feodora Maria; born 22 December 1978 at Hørsholm, Denmark) is the daughter of Prince Waldemar of Schaumburg-Lippe and his first wife, Anne-Lise Johansen.
 Princess Eleonore-Christine is a great-great-granddaughter of King Frederick VIII of Denmark.
 Under Danish law, she is barred from succession to the Danish throne, as she is not a descendant of Christian X.
 However, through her father, she is a descendant of King George II of Great Britain, and is therefore in line for the British throne.
Princess Caroline-Mathilde of Denmark (Danish: "Caroline-Mathilde Louise Dagmar Christine Maud Augusta Ingeborg Thyra Adelheid" ; 27 April 1912 – 12 December 1995) was a daughter of Prince Harald of Denmark and granddaughter of King Frederick VIII of Denmark.
 As the wife of Knud, Hereditary Prince of Denmark, she became Hereditary Princess of Denmark by marriage.

Question: Princess Caroline-Mathilde of Denmark was the wife of which younger son of Christian X?
Answer: Knud, Hereditary Prince of Denmark