The correct answer is: B. A perfect dam has been built by them across the river.
The sentence “They have built a perfect dam across the river” is in the active voice. The passive voice of this sentence is “A perfect dam has been built by them across the river.” This means that the dam was built by them, and not that the dam was built across the river.
The other options are incorrect because they do not accurately reflect the meaning of the original sentence. Option A, “Across the river a perfect dam was built,” is in the passive voice and does not specify who built the dam. Option C, “A perfect dam should have been built by them,” is in the subjunctive mood and expresses a wish or possibility. Option D, “Across the river was a perfect dam,” is in the simple past tense and does not specify who built the dam.
In conclusion, the correct answer is: B. A perfect dam has been built by them across the river.