The correct answer is option 1, Adrenaline. Adrenaline, also known as epinephrine, is a hormone that is produced by the adrenal glands and secreted into the bloodstream. It is a part of the body`s response to stress or danger, commonly known as the fight-or-flight response.

Adrenaline plays a crucial role in regulating the rate of heart beat. When the body perceives a threat, such as an emergency situation or a stressful event, the adrenal glands release adrenaline into the bloodstream. Adrenaline then acts on the heart by binding to specific receptors on the heart muscle cells. This binding activates a signaling pathway that leads to increased heart rate and force of contraction.

By increasing the heart rate, adrenaline ensures that oxygen and nutrients are delivered to the muscles and organs more efficiently, preparing the body for physical exertion or quick response. Once the threat or stress is resolved, the release of adrenaline decreases, and heart rate returns to normal.

It is important to note that while adrenaline controls the rate of heart beat in response to immediate stressors, the long-term regulation of heart rate is primarily controlled by the autonomic nervous system, specifically the sympathetic and parasympathetic branches.