What kind of animal is a blue whale?

The blue whale is classified as a mammal, which is the correct answer. This classification is based on several characteristics common to mammals. They are warm-blooded, have lungs for breathing air, and give birth to live young, all of which apply to blue whales. Additionally, like other mammals, blue whales possess mammary glands that produce milk to nourish their calves after birth.

In contrast, fish are cold-blooded, breathe underwater using gills, and typically lay eggs. Reptiles, which also lay eggs and are cold-blooded, have a different method of reproduction and physiological adaptations compared to mammals. Amphibians, while they do start life in water and have a stage where they possess gills, are not associated with the characteristics seen in blue whales. Therefore, identifying the blue whale as a mammal accurately reflects its biological and physiological traits.