Which part of the peripheral nervous system is responsible for involuntary body functions?

The autonomic nervous system is the correct choice because it is specifically responsible for regulating involuntary bodily functions. These functions include things like heart rate, digestion, respiratory rate, and functions of internal organs. It operates largely below the level of consciousness to control responses such as the heart beating and food moving through the digestive tract, which are vital for homeostasis.

The autonomic nervous system itself is divided into two main branches: the sympathetic and parasympathetic systems. The sympathetic system typically prepares the body for stressful or emergency situations ("fight or flight"), while the parasympathetic system helps to conserve energy and promote maintenance activities ("rest and digest"). The somatic nervous system, in contrast, governs voluntary movements and processes, such as muscle control and reflexes. The central nervous system encompasses the brain and spinal cord but does not directly regulate involuntary functions outside of its broader controlling role. Therefore, the autonomic branch is specifically designated for those involuntary actions, making it the appropriate answer.