Which of the following substances is classified as a CNS stimulant?

Caffeine is classified as a central nervous system (CNS) stimulant due to its ability to increase alertness, reduce fatigue, and enhance focus. It works primarily by blocking the action of adenosine, a neurotransmitter that promotes sleep and relaxation, thereby increasing the levels of other neurotransmitters such as dopamine and norepinephrine. This leads to heightened brain activity and improved cognitive performance.

In contrast, alcohol is a depressant, which means it slows down brain function and decreases alertness. Oxygen, while essential for survival and cellular respiration, does not have stimulant properties on the CNS. Valium, on the other hand, is a benzodiazepine that acts as a tranquilizer or depressant, promoting relaxation and reducing anxiety rather than stimulating the CNS. The classification of caffeine as a stimulant is well-supported by its widespread consumption and the physiological effects it produces.