Concept:In passive voice, the object of the active sentence becomes the subject, and the verb changes to a form of "be" plus the past participle. The tense must match the original action.
Explanation:The original sentence is in the simple past tense: "The engineers completed the structural analysis."
To convert to passive, make the object "the structural analysis" the subject.
The verb "completed" becomes "was completed" because the action happened in the past.
Add the original subject after "by": "by the engineers".
The time clause "before the design phase began" remains unchanged.
Now check the options: Option A uses "is completed" (present tense) – wrong tense.
Option B uses "was being completed" (past continuous) – incorrect; the original is simple past.
Option C uses "had been completed" (past perfect) – unnecessary and changes meaning.
Only option D uses "was completed" (simple past passive) – correct.
Answer:D. The structural analysis was completed by the engineers before the design phase began.