Your answer is B mammals because the sentences is sating that whales are mammals so the subject is mammals in this sentence also its not A odeon because the ocean can be a subject but not in this sentence its also not D because live is not a subject its just a word in a sentencewhen it came down to rather B, or C I choose C because whales is a thing, or a object but the subject is what their explaining the object is.
Your answer is B.