The correct answer is (a) shark. Sharks are the only fish that have three hearts. The first heart pumps blood to the gills, where it picks up oxygen. The second heart pumps blood to the rest of the body. The third heart pumps blood to the liver. Sharks have blue blood because their blood contains hemocyanin, a protein that carries oxygen. Hemocyanin contains copper, which gives it a blue color.
Option (b), blue whale, is incorrect. Blue whales are the largest animals on Earth, but they only have one heart.
Option (c), elephant, is incorrect. Elephants are the largest land animals on Earth, but they only have one heart.
Option (d), octopus, is incorrect. Octopuses are invertebrates, which means they do not have a skeleton. They also do not have a heart. Instead, they have a system of blood vessels that circulates blood throughout their body.