Why B? And why not the others?

Correct answer: B. The peregrine falcon

In its hunting dive, called a stoop, the peregrine falcon folds its wings and plummets at well over 320 km/h (200 mph) — one trained falcon was clocked at 389 km/h (242 mph) — making it the fastest animal of any kind. The cheetah is the classic trap answer: it is only the fastest runner on land, topping out near 100 to 110 km/h, roughly a third of a stooping falcon's speed. The sailfish holds the equivalent speed title in the water, and the pronghorn is North America's long-distance sprint champion, but neither comes close to the falcon's dive. The falcon survives this violence with remarkable adaptations: bony baffles in its nostrils tame the ram-air pressure, a third eyelid wipes its eyes clear mid-dive, and its body tolerates pull-out forces that would make a fighter pilot black out.

Source: US National Park Service & Cornell Lab of Ornithology (All About Birds): peregrine falcon stoop speed