My favourite animal is the dog. My favourite animals are dogs. Dogs are my favourite animals.

-C- A is incorrect because you said dogS B is incorrect because you said "animal" are C is the best answer. better to say : My favorite animal is the dog . *And note that you are talking about dogs in general so there is no need to say dogS. because you are not talking about a dog or many dogs.

animal 单数指一类物，我觉得My favorite animal is the dog 特指狗，前面animals的话，后面就接are the dog and the cat