Question: Thomas married Elizabeth, the widow of John Stewart, 2nd Earl of Buchan, was the daughter of which Scottish nobleman and warlord?

Context paragraphs:
James Stewart, 2nd Earl of Moray: James Stewart (or Stuart), 2nd Lord Doune and 2nd Earl of Moray (c. 1565 – 7 February 1591/2) was a Scottish nobleman, the son of James Stewart, 1st Lord Doune.  The 2nd Earl was murdered by George Gordon, Earl of Huntly, as the culmination of a vendetta.  Known as "the Bonnie Earl" for his good looks, he became the subject of a popular ballad, "The Bonnie Earl of Moray".

Matthew Stewart, 2nd Earl of Lennox: Matthew Stewart, 2nd Earl of Lennox (1460 – 9 September 1513, Flodden) was a prominent Scottish nobleman.  Stewart was the son of John Stewart, 1st Earl of Lennox, and Margaret Montgomerie, daughter of Alexander, Master of Montgomerie.

Archibald Douglas, 4th Earl of Douglas: Archibald Douglas, Duke of Touraine, Earl of Douglas, Earl of Wigtown, Lord of Annandale, Lord of Galloway, Lord of Bothwell, and 13th Lord of Douglas (1372–17 August 1424), was a Scottish nobleman and warlord.  He is sometimes given the epithet "Tyneman" (Old Scots: Loser), but this may be a reference to his great-uncle Sir Archibald Douglas.

John Stewart, Earl of Buchan: John Stewart, Earl of Buchan (c. 1381 – 17 August 1424) was a Scottish nobleman and soldier who fought alongside Scotland's French allies during the Hundred Years War.  In 1419 he was sent to France by his father the Duke of Albany, Regent of Scotland, with an army of 6,000 men.  Stewart led the combined Franco-Scottish army at the Battle of Baugé on 21 March 1421, where he comprehensively defeated the English forces, proving that the English could at last be beaten.

John Stewart, 3rd Earl of Atholl: John Stewart, 3rd Earl of Atholl (1507-1542) was the son of John Stewart, 2nd Earl of Atholl and Lady Janet Campbell, a daughter of Archibald Campbell, 2nd Earl of Argyll and Elizabeth Stuart.

Edward Montagu, 3rd Earl of Sandwich: Edward Montagu, 3rd Earl of Sandwich (10 April 1670 – 20 October 1729) was born in Burlington House, London, England to Edward Montagu, 2nd Earl of Sandwich and Lady Ann Boyle.  He was styled Viscount Hinchingbrooke from 1672 until his accession to the earldom in 1688.  On 8 July 1689 he married Elizabeth Wilmot, daughter of John Wilmot, 2nd Earl of Rochester and Elizabeth Malet.  They had two children, Elizabeth, who died as an infant, and Edward Richard Montagu, Viscount Hinchingbrooke (7 July 1692 – 3 October 1722), who predeceased his father.

William Douglas of Drumlanrig: Sir William Douglas, 1st of Drumlanrig (died 1427) was the illegitimate son of James Douglas, 2nd Earl of Douglas.  He fought against the English in the Hundred Years' War in France, where he was killed in action in 1427.  He married Elizabeth Stewart, daughter of Sir Robert Stewart of Durisdeer, and had one son, William Douglas, 2nd of Drumlanrig.

Thomas Stewart, Master of Mar: Thomas married Elizabeth, the widow of John Stewart, 2nd Earl of Buchan, who was daughter of Archibald Douglas, 4th Earl of Douglas and Margaret Stewart, Lady of Galloway.  They were required to obtain a marriage license, which was granted on 1 May 1427, due to their degrees of consanguinity and affinity.

John Stewart, 3rd Earl of Lennox: John Stewart, 3rd Earl of Lennox (c. 1490-4 September 1526, Linlithgow, West Lothian) was a prominent Scottish magnate.  He was the son of Matthew Stewart, 2nd Earl of Lennox, and Elizabeth Hamilton, daughter of James Hamilton, 1st Lord Hamilton and Mary Stewart, Princess of Scotland, daughter of King James II of Scotland.

Gilbert Kennedy, 2nd Earl of Cassilis: Gilbert Kennedy, 2nd Earl of Cassillis (died between 24 and 30 August 1527) was a Scottish nobleman, the son of David Kennedy, 1st Earl of Cassilis and Agnes, daughter of William Borthwick, 3rd Lord Borthwick.  He was married to Lady Isabel Campbell, daughter of Archibald Campbell, 2nd Earl Argyll and Elizabeth Stewart.

Supporting paragraphs: Thomas Stewart, Master of Mar Archibald Douglas, 4th Earl of Douglas
Answer: Archibald Douglas, 4th Earl of Douglas