The correct answer is D. Skin
Step-by-step explanation:
The words dermatologist, epiderm, hypoderm, taxidermist, and pachyderms share a common root (basic unit of the word) that is "derm". This root derives from the Greek word "derma" which means skin; this implies all of these words have to do or are related to skin. Indeed, dermatologists are experts that treat conditions on the skin; epidermis refers to the external layer of the skin; hypodermis refers to an inner layer of the skin; taxidermist refers to an expert that preserves animals by conserving the external layer of skin, and pachyderms include animals with thick skin.