Which of the following drugs is considered a stimulant?

Cocaine is classified as a stimulant because it primarily functions by increasing the levels of dopamine, a neurotransmitter associated with pleasure, in the brain's reward system. This leads to heightened alertness, energy, and euphoria, which are hallmark effects of stimulant drugs. Stimulants are known to elevate mood and enhance cognitive functions such as attention and focus.

In contrast, benzodiazepines and barbiturates are central nervous system depressants. They are used primarily for their sedative and anxiolytic properties, helping to reduce anxiety and promote sleep. Alcohol also acts as a depressant, affecting the central nervous system by slowing brain activity, which can lead to impaired judgment, decreased coordination, and slowed reflexes.

Therefore, cocaine stands out in this list as a stimulant, contrasting with the sedative effects of the other substances.