Sher Khan (later Sher Shah Suri) defeated Humayun at the Battle of Chausa in the year:

Sher Khan (later Sher Shah Suri) defeated the Mughal emperor Humayun at the Battle of Chausa on 26 June 1539.

About the battle:

• Chausa is a village on the bank of the Ganga, near Buxar, in present-day Bihar, close to the border with Uttar Pradesh.
• Humayun had gone to Bengal to deal with Sher Khan, and stayed at Gaur for several months, while Sher Khan cut off his communications with Agra.
• On his way back, Humayun's army camped at Chausa, and the two armies faced each other for some time.
• Sher Khan made a surprise attack at dawn, when the Mughal army was unprepared.
• The Mughal army was routed, and many soldiers drowned while trying to cross the Ganga.

Humayun's escape:

• Humayun himself narrowly escaped by swimming across the Ganga with the help of a water-carrier (bhishti) named Nizam.
• According to tradition, Humayun rewarded Nizam by allowing him to sit on the throne for half a day (or a few hours).

Results:

• After the victory, Sher Khan crowned himself and took the title "Sher Shah" (and "Sultan-i-Adil").
• He had coins struck and the khutba read in his name.
• Humayun returned to Agra and gathered a new army, but was decisively defeated by Sher Shah the next year, after which he lost his kingdom.

About Sher Shah Suri:

• He was an Afghan of the Sur tribe.
• He had served in the administration of Bihar, and rose through his ability to become the ruler of Bihar and Bengal.
• He ruled north India from 1540 to 1545.
• He was an able administrator, and introduced many reforms.
• He died in 1545 in an explosion during the siege of Kalinjar.

Humayun's exile:

• After losing his kingdom, Humayun went into exile and sought help from the Shah of Persia.
• He returned to India in 1555.

Why not the other options?

• ✗1537 was when Humayun moved against Sher Khan in the east.
• ✗1540 was the year of the next battle.
• ✗1545 was the year of Sher Shah's death.