Humayun had to flee from India after suffering defeat in the battle of Kannauj. The battle of Kannauj took place in 1540 between Humayun, the second Mughal emperor, and Sher Shah Suri, the ruler of the Sur dynasty. This battle was a major turning point in Indian history as it resulted in the loss of the Mughal Empire`s control over northern India and marked the rise of the Sur dynasty. Humayun`s defeat forced him to abandon his throne and seek refuge in Persia. However, he eventually managed to regain his empire after defeating Sher Shah`s son, Islam Shah, and reclaiming his kingdom in 1555. Therefore, option 4 - Kannauj is the correct answer.