Final answer:
Caffeine acts as a central nervous system stimulant by blocking the action of the neurotransmitter adenosine, preventing drowsiness and increasing alertness, which means the correct answer is B. inhibiting a specific neurotransmitter.
Step-by-step explanation:
The caffeine found in beverages like coffee and certain medicines functions as a central nervous system stimulant. It exerts its effect by inhibiting a specific neurotransmitter known as adenosine. Normally, adenosine binds to its receptors and promotes sleepiness or drowsiness. However, when caffeine is present, it serves as an antagonist to adenosine receptors, thereby blocking the action of adenosine. This prevents the onset of drowsiness and results in increased alertness and arousal. Since caffeine directly inhibits the function of a neurotransmitter by blocking its receptors, the correct answer is B. inhibiting a specific neurotransmitter.