Malcolm is studying alone in his room late at night when he hears a loud noise downstairs. His heartbeat increases significantly and his breathing becomes shallow. He wonders if a burglar has entered the house and decides to investigate. When he gets downstairs he discovers his cat has knocked over a plantstand. His body begins to relax and return to normal. Which part of his nervous system is responsible for returning Malcolm to a normal state?

A. Spinal cord
B. Somatic nervous system
C. Sympathetic nervous system
D. Parasympathetic nervous system