Concept:A past perfect active question becomes passive as: Had + new subject + been + past participle?Explanation:Active: "Had they completed the project before the deadline?" (past perfect).The object "the project" becomes the subject of the passive sentence.The tense stays past perfect, so "Had" stays at the front and "been" is added before the past participle "completed".The doer "they" becomes "by them".Passive: "Had the project been completed by them before the deadline?""Has" would change the tense to present perfect; "Were" and "Was" cannot go with "been completed" here.Answer:Option C (Had)