I . . . . . . . . a letter to her yesterday. A. had sent B. have sent C. was senting D. sent

The correct answer is D. sent.

The verb “send” is in the simple past tense. This is because the sentence is about a completed action that happened in the past. The other options are not in the simple past tense.

• Option A, “had sent,” is in the past perfect tense. This tense is used to talk about an action that happened before another action in the past. In this sentence, there is no other action in the past that the action of sending the letter happened before.
• Option B, “have sent,” is in the present perfect tense. This tense is used to talk about an action that happened at an unspecified time in the past or that is still happening. In this sentence, the action of sending the letter happened at a specific time in the past, so the present perfect tense is not appropriate.
• Option C, “was sending,” is in the past progressive tense. This tense is used to talk about an action that was happening at a specific time in the past. In this sentence, the action of sending the letter is not happening at a specific time in the past, so the past progressive tense is not appropriate.