Find out whether there is any grammatical error in below sentence. An animal / can be just as unhappy in a vast area / or in a small one

The correct answer is D. No error.

The sentence “An animal can be just as unhappy in a vast area or in a small one” is grammatically correct. The subject “animal” agrees with the verb “can be” in number, and the prepositional phrases “in a vast area” and “in a small one” are both necessary