Our task had been completed before sunset.

Explanation

This question is based on the concept of Active and Passive Voice. Understanding the voice is crucial to transforming sentences correctly while maintaining their original meaning and tense.

Voice: In grammar, voice refers to the form of a verb that shows whether the subject performs the action (active voice) or receives the action (passive voice).

Passive Voice Structure:
In passive voice, the focus is on the action or the recipient of the action rather than the doer. The structure generally follows:
Object + had been + past participle (V3) + (by + subject)

Original Sentence (Passive Voice): Our task had been completed before sunset.

Step-by-step Conversion to Active Voice:
1. Identify the subject: Implicit in passive voice. Likely "we".
2. Change "our task" to the object.
3. "Had been completed" is the past perfect passive. The active form of this is "had completed".
4. Keep the time expression "before sunset" as it is.

Correct Transformation:
We had completed our task before sunset.

Correct Answer: Option D: We had completed our task before sunset.