Why D? And why not the others?

Correct answer: D. The oceanic dolphins

The orca is the largest member of Delphinidae, the oceanic dolphin family — essentially a dolphin scaled up to nine metres and six tonnes, with the same toothed jaws, echolocation and tight-knit social pods as its smaller cousins. The word 'whale' in its name is the trap: it likely traces back to sailors calling them 'whale killers' after watching pods hunt great whales, and the phrase later flipped around. Baleen whales are a tempting choice because of that name, but orcas carry rows of interlocking conical teeth and no baleen plates at all. The tall triangular dorsal fin gives an orca a shark-like silhouette slicing through the water, yet sharks are fish while orcas are warm-blooded, air-breathing mammals. And seals and sea lions are no relation either — they are among the orca's favourite prey.

Source: NOAA Fisheries: killer whale species profile — largest member of the dolphin family Delphinidae