The correct answer is option-1, Thyroxine. Thyroxine is a hormone produced by the thyroid gland and it plays a crucial role in regulating several bodily functions, including the heart rate. Thyroxine increases the rate at which the heart beats, leading to an increase in heart rate. It does so by affecting the sensitivity of the heart to other hormones like adrenaline and noradrenaline, which further stimulate the heart. This increase in heart rate helps to ensure adequate blood circulation throughout the body. Therefore, Thyroxine is the hormone that stimulates heart beat.