Final answer:
Caffeine acts as a competitive antagonist to adenosine receptors, generally leading to increased alertness and decreased sleepiness by disrupting the normal inhibitory function of adenosine on excitatory neuronal networks. The correct option is a).
Step-by-step explanation:
Caffeine is a competitive antagonist of adenosine receptors, which normally block the activity of excitatory networks. This means that the correct answer to the student's question is a) Adenosine; Excitatory.
Caffeine's function as a central nervous system stimulant is primarily due to its ability to block adenosine receptors, which are responsible for promoting sleep and suppressing arousal.
When caffeine inhibits these receptors, it results in decreased sleepiness and increased wakefulness, contributing to heightened alertness and arousal. Option a) is the correct one.