Final answer:
Epinephrine and norepinephrine are produced by the adrenal glands and function as neurotransmitters in the sympathetic nervous system, making option C the correct answer.
Step-by-step explanation:
The substances produced by the adrenal glands that function as neurotransmitters and stimulate activity at the synapses in the sympathetic nervous system are epinephrine and norepinephrine. These chemicals, also known as adrenaline and noradrenaline respectively, are part of the adrenergic system. While acetylcholine indeed is a neurotransmitter, it is more associated with the parasympathetic nervous system and at neuromuscular junctions, rather than the sympathetic nervous system that is the focus of the question. Therefore, the correct answer to the student's question is: (C) epinephrine and norepinephrine.