Which part of the brain is responsible for regulating involuntary functions such as breathing and heart rate?

The medulla oblongata, located in the brain stem, is responsible for regulating involuntary functions such as breathing and heart rate. It contains vital centers that control these essential functions to maintain life. The cerebrum is primarily involved in higher brain functions like thinking and voluntary movements; the cerebellum plays a crucial role in coordinating movement and balance; and the frontal lobe is responsible for functions such as decision-making, problem-solving, and voluntary muscle movements. Therefore, the correct answer is the medulla oblongata (Choice C).