Active-to-passive voice transformation (present perfect) Original sentence: They have built a perfect dam across the river. Choose the correctly formed passive-voice sentence preserving tense and complements.

Concept/Approach
Present perfect passive: "has/have been + past participle". Keep the place complement "across the river".

Option analysis
Correct: "A perfect dam has been built by them across the river."(a) Changes tense to simple past and distorts word order.(c) Modal "should have been" alters meaning (obligation/expectation).(d) Copular description, not a passive transformation of the action.

Final Answer
A perfect dam has been built by them across the river.