Which system is comprised of the brain, spinal cord, and nerves and regulates all of the body's activities?

The nervous system is the correct answer because it is responsible for regulating and coordinating all of the body's activities through a complex network that includes the brain, spinal cord, and nerves. The brain serves as the control center, processing sensory information and sending signals throughout the body via the spinal cord and peripheral nerves. This system enables various functions, such as voluntary movements, reflexes, and the processing of external stimuli, essential for maintaining homeostasis and responding to changes in the environment.

In contrast, the endocrine system primarily controls bodily functions through hormones, affecting growth, metabolism, and reproduction, but it does not regulate activities in the immediate, rapid response manner that the nervous system does. The circulatory system facilitates the distribution of blood, nutrients, and oxygen but does not directly control the body's responses or activities. The muscular system is responsible for movement but is dependent on signals from the nervous system to initiate those movements. Thus, the nervous system is fundamentally the regulatory system that oversees and integrates all these processes.