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

The integumentary system is the correct answer because it encompasses all the components mentioned: skin, mucous membranes, hair, and nails. This system serves multiple crucial functions, including protection, regulation of temperature, sensation, and the synthesis of vitamin D. It acts as a barrier to protect the internal organs from external dangers, such as pathogens and physical injuries. The skin is the largest organ of the body and plays a vital role in overall health and well-being.

In contrast, the muscular system is focused primarily on movement and the manipulation of the body through muscular contractions. It does not include the skin or related structures. The skeletal system is made up of bones that provide support, structure, and protection for the body, but it does not encompass any of the integumentary components. Lastly, the nervous system is responsible for controlling and communicating messages throughout the body, which involves nerve cells and brain structures, not the skin or other integumentary system parts. Therefore, the integumentary system is clearly the one that includes all the specified elements.