The correct answer is option 4: Adrenaline. Adrenaline is a hormone that is released by the adrenal glands in situations of stress or excitement. When released into the bloodstream, adrenaline acts on various target organs and tissues, including the heart.

Adrenaline exerts its effects by binding to specific receptors on the surface of heart cells called cardiac muscle cells. When adrenaline binds to these receptors, it triggers a series of cellular events that lead to an increase in the strength and rate of heart contractions. This increased contraction strength and rate result in an elevated heartbeat.

On the other hand, the other options listed are not involved in increasing the heartbeat. Insulin is a hormone produced by the pancreas that helps regulate blood sugar levels. Melatonin is a hormone involved in sleep-wake cycles. Thyroxine is a thyroid hormone that regulates metabolism. None of these hormones directly influence the heartbeat.