Your question's answer should be (a). Your (b) answer is not wrong either,but it would be more suitable to be used when you're referring to two or more animals -What is/are your favorite animal/-s? -My favorite animal is dog or -My favorite animals are cats and dogs/My favorite animals are dogs Same goes for your second answer -What is/are your favorite fruit/-s? -My favorite fruit is banana or -My favorite fruits are banana and apple/My favorite fruits are apples