The correct option is D Casting off old skin.
Moulting is the manner in which an animal routinely casts off a part of its body at specific points in its life cycle. Moulting can involve shedding the epidermis (skin), pelage (hair, feathers, fur, wool), or other external layer. In some species, other body parts may be shed, for example, wings in some insects or the entire exoskeleton in arthropods.