The correct answer is "will have finished." This answer is in the future perfect tense. The future perfect tense is used when we want to talk about an action that will be completed at some point in the future before another action takes place. In this sentence, the action that will be completed is the finishing of our homework, and the second action is the time of 9 o'clock.


The future perfect tense is formed using "will have" + the past participle of the verb. For example:

- I will have eaten breakfast by the time you wake up.
- They will have finished the project before the deadline.


The future perfect tense is used when:

- We want to talk about an action that will be completed before a specific time or event in the future. In this sentence, the specific time is 9 o'clock.
- We want to emphasize the completion of an action before another action takes place.


In conclusion, the correct answer to the question "By 9 o'clock, we ___________ (finish) our homework" is "will have finished." This answer is in the future perfect tense and emphasizes the completion of our homework before the time of 9 o'clock.