Why B? And why not the others?

Correct answer: B. A venomous spur on each hind ankle

Male platypuses carry a sharp, hollow spur on each hind ankle, connected by a duct to a venom gland in the thigh; it is used mainly against rival males during breeding season and produces intense, long-lasting pain in humans, though it is rarely life-threatening. A bite delivered through grooved teeth is how many venomous snakes and lizards deliver venom, but the platypus's rubbery bill contains no fangs or venom ducts at all — it is a sensory organ used for electroreception while hunting underwater, not a weapon. Venomous saliva also doesn't fit the platypus's anatomy, since its bill has no venom-producing glands. There is no venom-coated fur mechanism documented in any mammal; the platypus's only venom-delivery point is the ankle spur, which is why it is often singled out as one of the handful of venomous mammals alongside a few shrews and solenodons.

Source: Australian Museum: platypus venom and spur; San Diego Zoo Wildlife Alliance: platypus fact sheet.