Which of the following is considered a central nervous system depressant?

Alcohol is classified as a central nervous system depressant due to its effects on brain function and behavior. It slows down brain activity, leading to a reduction in anxiety, inhibition, and overall mental acuity. When consumed, alcohol enhances the effects of the neurotransmitter gamma-aminobutyric acid (GABA), which further depresses neural activity. This results in the sedative effects commonly associated with alcohol consumption, such as relaxation, drowsiness, and impaired coordination.

Caffeine, in contrast, is a stimulant that increases alertness and energy levels. Nicotine, although it can have both stimulant and depressant effects depending on the dosage and individual response, is primarily considered a stimulant because it activates the brain's reward pathways, creating feelings of pleasure and alertness. Serotonin is a neurotransmitter that regulates mood, and while it plays a role in many bodily functions, it does not fit into the category of central nervous system depressants. Hence, alcohol stands out as the correct choice.