Which of the following substances is NOT a stimulant?

Alcohol is classified as a depressant rather than a stimulant. While stimulants are substances that enhance activity in the central nervous system, leading to increased alertness, energy, and focus, alcohol has the opposite effect. It slows down brain activity and impairs cognitive functions, leading to decreased inhibition and coordination.

On the other hand, meth, cocaine, and ecstasy are all stimulants. They elevate mood, increase energy and alertness, and enhance feelings of well-being. These substances work by increasing the levels of certain neurotransmitters in the brain, primarily norepinephrine and dopamine, which are associated with pleasure and the body's fight-or-flight response. This is why alcohol stands out as the substance that does not share this stimulating effect, making it the correct answer to the question.