The option "The dolphin is not (a) fish." The definite article "a" is required to indicate that we are referring to the general class of animals known as fish as we are referring to a single unit. In the option: "Dolphins are not fish." We are referring to dolphins in plural, encompassing the entire family of dolphins, so fish is also plural, and hence does not require the definite article "a", which would suggest that it was singular. Note that, in English, the word "fish" can indicate both singular and plural. Confusingly, the word "fishes" can also be used in certain circumstances to indicate the plural.