Which system includes the skin, mucous membranes, hair, and nails?

The integumentary system is the correct answer because it comprises the skin, mucous membranes, hair, and nails, all of which serve vital protective and functional roles in the body. The skin acts as a barrier, protecting internal organs from damage and pathogens. It also plays a key role in regulating body temperature and perceiving sensations. Mucous membranes line various cavities and organs, providing lubrication and protecting against infection. Hair contributes to insulation and protection, while nails protect the sensitive tips of fingers and toes.

In contrast, other systems mentioned do not encompass these elements. The muscular system primarily focuses on muscles and their functions in movement. The nervous system deals with the brain, spinal cord, and nerves, facilitating communication between different parts of the body. The endocrine system involves glands that secrete hormones, influencing various bodily functions but does not include skin or hair components. Therefore, the integumentary system stands out as the group that includes all these specific elements.