The correct option is A him
Let’s examine the sentence: “Tom's mother sent ____ a letter.” The sentence has verb ‘sent’ that requires an object in the form of objective pronoun. The personal pronouns ‘me, you, us, him, her, it, and them’ are called objective pronouns because they act as the objects of verbs and prepositions. S, Option A is the correct answer. All the other options would make the sentence meaningless and so, cannot be used.