Answer:
A. By the end of the period, all the students had completed the exam.
Explanation:
The past perfect tense is the tense which shows that the action has been finished at some point of time in the past. The action here gets completed before something else had happened. The tense gets formed by adding 'had' with the past participle of the verb respectively. In the above sentences, sentence A is the sentence which contains a past perfect verb.