Explanation:

To understand why the correct answer is "will have finished," we need to look at the context of the sentence and the use of verb tenses.

1. Future perfect tense:
The verb tense used in the correct answer is the future perfect tense. This tense is used to describe an action that will be completed before a specific point in the future. In this case, the specific point is "9 o'clock."

2. Time marker:
The time marker "By" indicates that the action of finishing the homework will be completed before the specified time of 9 o'clock.

3. Subject and verb agreement:
The subject of the sentence is "we," which is a first-person plural pronoun. The corresponding verb form for "we" is "will have finished" in the future perfect tense.

4. Structure:
The structure of the future perfect tense is formed by using the auxiliary verb "will" followed by "have" and the past participle form of the main verb.

5. Past participle of "finish":
The past participle form of "finish" is "finished." When combined with "will have," it forms the future perfect tense.

Example:

Let's use the sentence "By 9 o'clock, we will have finished our homework" in a context:

Context:
You and your friends are working on a group project, and you plan to meet at 9 o'clock to discuss and finalize the project. You are confident that everyone will complete their assigned tasks before the meeting.

Sentence:
By 9 o'clock, we will have finished our homework.

Explanation:
This sentence means that before 9 o'clock, all of you will complete your homework. It suggests that the homework will be completed and ready for discussion by the specified time.

Overall, the correct answer "will have finished" in the future perfect tense accurately conveys the completion of the homework before the specified time of 9 o'clock.