Which system primarily controls involuntary body functions such as heart rate and digestion?

The autonomic nervous system is the correct choice because it specifically regulates involuntary body functions that are essential for maintaining homeostasis. This system is responsible for controlling processes such as heart rate, digestion, respiratory rate, pupillary response, urination, and sexual arousal, without requiring conscious thought or effort from an individual.

In contrast, the central nervous system, which includes the brain and spinal cord, plays a crucial role in processing and integrating sensory information and coordinating motor output, but it does not specifically manage involuntary functions. The peripheral nervous system encompasses all the nerves outside the central nervous system and includes both the autonomic and somatic nervous systems, making it a broader categorization that includes voluntary movements as well.

The somatic nervous system, another component of the peripheral nervous system, specifically governs voluntary movements and transmits sensory information to the central nervous system. Thus, while it is vital for activities such as moving muscles, it does not regulate involuntary functions like heart rate and digestion.