The correct option is C Assertion is correct but Reason is incorrect.
Whales are mammals because they give birth to young ones, they provide milk for their young ones and they possess the diaphragm. Viviparity and body hairs are the property of mammals but the defining property of mammals are the mammary gland. Platypus are the egg-laying mammal, and some aquatic mammals do not possess body hairs. Thus, the correct answer is option C.