answer:
The correct answer to complete the sentence is "A whale is a mammal, not a fish."
In this sentence, the definite article "the" is not appropriate because it implies that there is a specific whale that is being referred to. Since the sentence is making a general statement about whales in general, the indefinite article "a" is used to indicate any whale. The use of "an" is not correct because "whale" begins with a consonant sound, so we use "a" instead. Therefore, the correct answer is option "a."
real