Final answer:
Caffeine affects the central nervous system by acting as an antagonist to adenosine receptors, preventing drowsiness and increasing alertness and wakefulness.
Step-by-step explanation:
Caffeine produces its effects by directly influencing the functioning of the c) Adenosine system. Caffeine is a central nervous system stimulant that is classified in the methylxanthine class. It is known to block the action of adenosine on its receptors, preventing the onset of drowsiness which adenosine induces.
Since adenosine typically promotes sleep by suppressing the activity of the neurochemicals associated with wakefulness, caffeine's antagonistic effects on the adenosine receptors result in reduced sleepiness and increased alertness. Additionally, by blocking adenosine receptors, caffeine indirectly increases adrenergic activity, as adenosine would otherwise suppress the release of neurotransmitters like norepinephrine at synapses.