To solve this question, the user needs to know the different verb tenses and their uses. This sentence is in the future perfect tense, which is used to show that an action will have been completed before a specific point in the future.

Now, let's go through each option and explain why it is right or wrong:

A. finish: This option is incorrect because "finish" is in the simple present tense, which is not appropriate for this sentence. The sentence is in the future perfect tense, indicating that the action will be completed in the future.

B. will have finished: This option is correct. "Will have finished" is in the future perfect tense, which is appropriate for this sentence. It shows that the action of finishing homework will be completed before the time the person arrives.

C. will finish: This option is incorrect because "will finish" is in the simple future tense, which does not convey the sense of completion required by the sentence.

D. were finished: This option is incorrect because "were finished" is in the past tense, which is not appropriate for this sentence. The sentence is in the future perfect tense, indicating that the action will be completed in the future.

Therefore, the answer is: B. will have finished