From the given alternatives, choose the one which best expresses the sentence in Active/Passive voice.
The MCD has built a perfect dam across the river.

A perfect dam has been built by the MCD across the river.

The correct answer is A perfect dam has been built by the MCD across the river.

The original sentence is in active voice, with the subject "MCD" and the verb "has built". To change it to passive voice, we make the object of the active sentence, "dam", the subject of the passive sentence. The verb "has built" becomes "has been built", and we add the preposition "by" to show who is doing the action. This gives us the passive voice sentence "A perfect dam has been built by the MCD across the river".

The other options are incorrect.

• Across the river a perfect dam was built is in the active voice, but it changes the meaning of the sentence. The original sentence means that the MCD built a perfect dam, but the active voice sentence means that someone built a perfect dam across the river.
• A perfect dam should have been built by the MCD is in the modal passive voice. This sentence is used to express a wish or a regret. It is not appropriate for the original sentence, which is a statement of fact.
• Across the river was a perfect dam is a sentence fragment. It does not have a subject or a verb.