Which of the following is a common depressant?

A common depressant is a substance that slows down the central nervous system and can produce effects such as relaxation, drowsiness, and decreased inhibition. In this context, alcohol fits this definition perfectly. It is widely known for its ability to create a calming effect on the body and mind, often leading to reduced anxiety and social inhibition.

The other substances listed serve different purposes and do not primarily act as depressants. For example, Prozac, which is an antidepressant medication, works by increasing levels of serotonin in the brain to help alleviate symptoms of depression and anxiety, thereby promoting a more balanced mood rather than inducing a depressant effect. Caffeine, while often considered a stimulant that increases alertness and energy levels, clearly opposes the characteristics of a depressant. Lastly, LSD is a hallucinogenic drug known for altering perception, mood, and cognitive processes, which also diverges from the depressant category. Thus, alcohol stands out as the clear example of a common depressant.