The correct answer is: C. had sent
The sentence is in the past perfect tense, which is used to talk about an event that happened before another event in the past. In this case, the first event is “you gave answers” and the second event is “we received them.” The past perfect tense is formed with the past participle of the verb (sent) and the auxiliary verb had.
The other options are incorrect because they do not agree with the tense of the sentence. Option A, sent, is in the simple past tense, which is used to talk about an event that happened at a specific time in the past. Option B, will send, is in the future tense, which is used to talk about an event that will happen in the future. Option D, was sending, is in the past progressive tense, which is used to talk about an event that was happening at a specific time in the past.
Here is a diagram that shows the tenses of the verbs in the sentence:
| Event | Tense |
|—|—|
| You gave answers | Past tense |
| We received them | Past perfect tense |
I hope this explanation is helpful!