The correct answer is A. Adrenaline.
Adrenaline is a hormone that is released in response to stress or danger. It causes the heart to beat faster, the blood vessels to constrict, and the muscles to tense. This prepares the body for a fight-or-flight response.
Acetylcholine is a neurotransmitter that is involved in muscle contraction and the transmission of nerve impulses. It does not have a direct effect on the heart rate.
Thyroxine is a hormone that is produced by the thyroid gland. It helps to regulate metabolism and growth. It does not have a direct effect on the heart rate.
Pituitrin is a hormone that is produced by the pituitary gland. It has a variety of effects on the body, including stimulating the production of milk and regulating blood sugar levels. It does not have a direct effect on the heart rate.
In conclusion, the hormone that accelerates the rate of heartbeat is adrenaline.