The correct answer is: The hikers will have got so tired
by the time they reach the top of the hill that I’m sure they’ll first want to sit down for a rest.The sentence is in the future perfect tense, which is used to talk about actions that will be completed before a specific time in the future. In this case, the specific time is when the hikers reach the top of the hill. The future perfect tense is formed with the auxiliary verb “will have” and the past participle of the main verb. In this case, the main verb is “get,” so the past participle is “got.”
The other options are incorrect because they do not use the future perfect tense. Option A, are getting/will have reached, is in the present progressive tense. Option B, had got/reached, is in the past perfect tense. Option C, have got/are reaching, is in the present perfect progressive tense. Option D, were getting/had reached, is in the past progressive tense.
The future perfect tense is the most logical tense to use in this sentence because it focuses on the completion of the action (the hikers getting tired) before a specific time in the future (when they reach the top of the hill). The other tenses do not focus on the completion of the action, so they are not appropriate for this sentence.