Final answer:
When activated by the sympathetic nervous system, the adrenal medulla secretes epinephrine, also known as adrenaline, which plays a critical role in the body's fight-or-flight response by increasing various physiological activities. The correct option is A.
Step-by-step explanation:
When the adrenal medulla is activated by the sympathetic nervous system, epinephrine (A) is secreted. This process is a part of the body's fight-or-flight response, which prepares the body to face or escape threats. The adrenal medulla releases both epinephrine and norepinephrine in response to stress, fear, anger, exercise, and hypoglycemia.
These catecholamines circulate in the plasma, and they have various effects, including increasing heart rate, blood pressure, and blood glucose levels. They also accelerate the breakdown of glucose in skeletal muscles and fats stored in adipose tissue.
There is no parasympathetic stimulation to the adrenal medulla; it is primarily regulated by the sympathetic nervous system.