A. We completed our task before sunset.
B. We have completed our task before sunset.
C. We complete our task before sunset.
D. We had completed our task before sunset.
The given sentence is in passive voice and it is in Past Perfect Tense.
To convert it into active voice, we just remove been from the given sentence and object (our)will be change into subject (We).
Rule :
Subject + had + V3 + Other agents.
