Final answer:
Option (d), Thyroxine or T4 is a thyroid hormone that not only affects the body's metabolism, temperature, and growth but also significantly influences the heart rate through its interaction with catecholamines.
Step-by-step explanation:
Thyroxine, also known as tetraiodothyronine or T4, is a thyroid hormone that significantly influences the body's metabolism, temperature, and growth. Additionally, thyroxine impacts the heart rate by increasing the body's sensitivity to catecholamines such as epinephrine and norepinephrine.
This sensitivity is due to the upregulation of receptors in the blood vessels, which when in excess, can accelerate heart rate, strengthen the heartbeat, and elevate blood pressure. Therefore, the answer to the student's question is d. heart rate.