What system regulates the body's responses to internal and external stimuli?

The nervous system is the correct answer because it plays a crucial role in regulating the body's responses to various internal and external stimuli. This system comprises the brain, spinal cord, and nerves, which work together to detect changes in the environment, process information, and coordinate appropriate responses. It allows for quick reactions to stimuli such as touch, sound, or light and influences bodily functions by transmitting signals throughout the body.

In contrast, the digestive system primarily focuses on the breakdown and absorption of nutrients from food, the muscular system is involved in movement and physical activity, and the circulatory system is responsible for the transportation of blood, oxygen, and nutrients throughout the body. While these systems are essential for overall functioning, they do not primarily regulate responses to stimuli as the nervous system does.