Fill in the blank with the correct form of the verb to complete the following sentence: He ________ (begin)to collect small stones on his way back to the ashram from his regular daily walks.

The correct answer is: C. began

The sentence is in the past simple tense, so the verb must be in the past simple form. The other options are not in the past simple tense.

A. begins is in the present simple tense.
B. has begun is in the present perfect tense.
D. had begun is in the past perfect tense.

The sentence is about a specific event that happened in the past, so the past simple tense is the most appropriate tense to use.