Change the voice: The engineers have built a perfect dam across the stream.

A perfect dam across the stream has been built by the engineers.

A perfect dam across the stream was built by the engineers.

A perfect dam across the stream has built by the engineers.

None of these

Tense: Present Perfect (have built) → Passive: has been built

Active voice: The engineers have built a perfect dam across the stream.

Passive voice: A perfect dam across the stream has been built by the engineers.

"A perfect dam" is singular → has.