Fill in the blank with the most appropriate word. Ask him when the engineers ___ finish the talks.

The correct answer is: will.

The sentence is in the future tense, so the verb “finish” should be in the future tense as well. The other options are not in the future tense.

• Has is in the present perfect tense. It is used to talk about something that happened in the past but has an effect on the present. In this sentence, it would imply that the engineers have already finished the talks, which is not what the question is asking.
• Had is in the past perfect tense. It is used to talk about something that happened before another event in the past. In this sentence, it would imply that the engineers had finished the talks before something else happened, which is not what the question is asking.
• Have is in the present tense. It is used to talk about something that is happening now or that happens regularly. In this sentence, it would imply that the engineers are finishing the talks now, which is not what the question is asking.

Therefore, the only option that makes sense is will. It is in the future tense and it does not imply anything about the past or present.