Answer:
Mammals are different from fish, reptiles or birds
Explanation:
This is the correct answer because since dolphins are mammals, they are very different from the other types of animals such as fish, reptiles, or birds; this is emphasized in this paragraph ;)
I think it is C because A is missing commas, B doesn't need them and D doesn't need one.
I believe it would be to inform, the particular excerpt isn't really informing about anything, and its not persuading the reader to do anything, simply attempting to entertain the reader with a story.