Why Correct: Nadir Shah invaded India in 1739, defeating Mughal Emperor Muhammad Shah at the Battle of Karnal and sacking Delhi.
Distractor Analysis: 1738 marks Nadir Shah's campaigns in Afghanistan before entering India. 1740 was the year after the invasion when Nadir Shah returned to Persia. 1741 saw no major invasion of India by Nadir Shah.
Takeaway: Nadir Shah took the Koh-i-Noor diamond and the Peacock Throne from Delhi, severely weakening the Mughal Empire.