Answer:
Future perfect tense.
Step-by-step explanation:
The future perfect tense is when the action will have already been done at some point in the future. It is denoted by the use of "will' + "have" + the past participle form of the verb.
So, the sentence provided in the question
They will have finished (past participle form or third form of the verb 'finish).
is in the future perfect tense.
Therefore, the presence of "will' + "have" and the subsequent third form of the verb "finish" clearly shows that it is in the future perfect tense form.