Why C? And why not the others?

Correct answer: C. The skin

The skin is the body's largest organ, covering roughly 1.5 to 2 square meters and weighing around 4 kilograms, which can amount to nearly a tenth of your body mass. It is easy to forget skin is an organ at all, yet it is a three-layered structure that blocks pathogens and ultraviolet light, regulates temperature, and manages water loss. The liver is the classic tempting answer because it is the largest internal organ, at about 1.5 kilograms. The lungs impress with their vast internal gas-exchange surface, but that area is folded away inside a much lighter organ, and the large intestine, despite its name, is a relatively short tube.

Source: NIH StatPearls: Anatomy, Skin (Integument), Epidermis