Concept:A past perfect active question is changed into passive voice using:
Had + subject +
been + past participle.
Explanation:The active sentence is "Had they completed the project before the deadline?"
This is in the past perfect tense, and "Had" is the helping verb.
In a passive sentence, the object of the active sentence becomes the new subject.
Here, the object is "the project", so it moves to the front of the sentence.
The tense must remain past perfect, so "Had" stays at the beginning.
The word "been" is added after the subject to show the passive form.
The main verb keeps its past participle form, giving "been completed".
The original subject "they" moves to the end as the agent and becomes "them" after "by".
Thus, the passive question becomes: "Had the project been completed by them before the deadline?"
The word needed to fill the blank is therefore "Had".
Answer:Had the project been completed by them before the deadline?
Correct option: C. Had