The correct answer is "Both A and R are true, and R is the correct explanation of A."

Assertion (A) states that Lord Linlithgow described the August Movement of 1942 as the most serious rebellion since the Sepoy Mutiny. This implies that the August Movement was considered a significant and impactful rebellion.

Reason (R) explains that there was a massive upsurge of the peasantry in certain areas. This indicates that the movement involved a widespread and substantial participation of the rural population.

The reason provided (R) offers an explanation for why the August Movement was considered a serious rebellion. The massive upsurge of the peasantry in certain areas indicates the scale and intensity of the movement, which contributed to its significance.

Therefore, both the assertion (A) and the reason (R) are true, and the reason correctly explains the assertion.