Assertion (A): Lord Linlithgow described the August Movement of 1942 as the most serious revolt after the Sepoy mutiny. Reason (R): Peasants joined the movement in large number in some places.

The correct option is A because both statements are historically accurate and the reason directly justifies the assertion.

Assertion A is true because Lord Linlithgow, the then Viceroy of India, wrote a telegram to Prime Minister Winston Churchill describing the Quit India Movement also known as the August Movement as by far the most serious rebellion since that of 1857.

Reason R is true because the movement saw massive participation from the peasantry, especially in regions like Bihar, Eastern Uttar Pradesh, and Bengal. The Reason is the correct explanation of the Assertion because it was precisely this large scale mass participation and the violent nature of the rural uprisings which included attacking police stations and sabotaging rail lines that made the British authorities perceive it as a threat comparable to the 1857 mutiny.