Option 1 states that both Assertion (A) and Reason (R) are true, and that R is the correct explanation of A.

To evaluate this option, let`s examine both A and R.

Assertion (A) states that Buddhism became very popular in India during the lifetime of its founder. This is generally true. Siddhartha Gautama, the founder of Buddhism, gained a significant following during his lifetime and Buddhism spread rapidly in India.

Reason (R) states that Buddha preached in the language of the people and did not harp on the caste system. This statement is also true. Buddha preached in the common language of the people and emphasized the importance of personal spiritual development instead of focusing on the caste system, which was a dominant social structure in ancient India.

Now we need to determine if R is the correct explanation of A. In this case, it can be argued that R does explain why Buddhism became popular in India during the lifetime of its founder. By preaching in the language of the people and detaching from the caste system, Buddha attracted a large following from various social backgrounds and contributed to the popularity of Buddhism.

Therefore, based on the provided information, it can be concluded that both A and R are true, and R is indeed the correct explanation