Although Dr. Singh had several assistants during his long investigations of Tumni Islands, the bulk of research was done by ................... alone

A: This reflexive pronoun is used when the subject and the object of the verb refer to the same person. However, in the sentence provided, "himself" doesn't fit because it's not used as the object of the verb.

B: This is a subject pronoun and doesn't fit the sentence grammatically. "The bulk of research was done by he alone" is incorrect; it should be "by him alone."

C: This is a possessive pronoun, used to show ownership. "The bulk of research was done by his alone" doesn't make sense in this context.

D: This is the correct choice. "Him" is an object pronoun, which is used as the object of the verb "done" in the sentence.

Therefore, Option D is the correct answer.