(d) The brain is responsible for all the actions given in the options,viz., thinking, regulating the heartbeat and balancing the body.It is the central organ of the human nervous system. The brain consists of the cerebrum, the brain stem and the cerebellum. It controls most of the activities of the body,processing, integrating and coordinating the information it receives from the sense organs,and making decisions as to the instructions sent to the rest of the body.