Marine mammals are endothermic. I think that's the correct answer 'cause they can produce their own body heat (which therefore means that they are warm-blooded) and they can keep at a stable temperature even in harsh conditions. Plus, whales don't have gills b/c they are mammals (and mammals don't have gills), not all mammals have blubber (it's mainly penguins and seals), marine mammals can stay underwater for a long amount of time...but they eventually need to come up for air, and baleen acts like a "filter"; it "filter" and separates the water from the krill/microorganisms.
Hope this helps! :)