The addition of which of the following sentences would create an effective paragraph? dogs and cats are both fantastic pets. dogs are loving and playful. cats are clean and self-sufficient. dogs are sometimes hyper. (select a, b, c, or d and place here.) both are effective communicators in their own right. either one would make any lucky owner happy.

Assuming this is the paragraph:
"Dogs and cats are both fantastic pets. Dogs are loving and playful. Cats are clean and self-sufficient. Dogs are sometimes hyper. ___  Both are effective communicators in their own right. Either one would make any lucky owner happy,"
the correct answer is Cats are clean and self-sufficient. 
In the paragraph, you are supposed to show why both dogs and cats are good pets. First you talked about dogs, which means you should mention something about cats too. This is why that is the correct answer.