The correct answers are By the time the guests arrive, we will have baked 100 pizzas. and We will have finished decorating the dining hall before 6:00 PM.

The future perfect tense is formed with will + have + past participle (e.g., will have baked, will have finished). It is used to talk about an action that will be completed before a specific time in the future.

"Will have bake" is incorrect because "bake" is not the past participle. "Will had baked" is incorrect because the auxiliary verb after "will" must always be the base form "have", never "had".