The correct answer is C. to.
The word “accustomed” means “habituated by practice or experience.” In this sentence, the speaker is saying that the man is used to chewing tulsi leaves. This is evident in the fact that he does it even when he is delivering a lecture, which is a time when most people would not be chewing on anything.
The other options are not correct because they do not make sense in the context of the sentence. For example, the word “about” means “concerning” or “relating to.” In this sentence, it would not make sense to say that the man is accustomed about chewing tulsi leaves. The word “of” means “belonging to” or “coming from.” In this sentence, it would not make sense to say that the man is accustomed of chewing tulsi leaves. The word “at” means “in the position of” or “on the point of.” In this sentence, it would not make sense to say that the man is accustomed at chewing tulsi leaves.