The correct answer is option 2: Sympathetic nerves. The rate of heart beat is controlled by the autonomic nervous system, which is divided into two branches - the sympathetic nervous system and the parasympathetic nervous system. The sympathetic nervous system is responsible for activating the fight-or-flight response in the body, and one of its effects is to increase the heart rate. This is achieved by releasing norepinephrine from sympathetic nerve fibers that innervate the heart. Norepinephrine binds to receptors on the heart muscle, causing it to contract more frequently and forcefully, resulting in an increased heart rate. On the other hand, the parasympathetic nervous system has the opposite effect and slows down the heart rate. Therefore, option 2, Sympathetic nerves, increases the rate of heart beat.