Final answer:
Methamphetamine is a stimulant that increases alertness and wakefulness, while heroin, alcohol, and barbiturates are depressants that have a calming effect on the brain.
Step-by-step explanation:
Stimulants are a type of psychoactive drug that stimulates the brain and increases alertness and wakefulness. Among the options given, methamphetamine belongs to the class of stimulant drugs. Other common stimulants include caffeine, nicotine, and amphetamines like Adderall. In contrast, substances like heroin and marijuana are categorized differently; heroin is a depressant and marijuana has depressant, stimulant, and hallucinogenic properties depending on its use and user. Alcohol and barbiturates are also depressants, not stimulants, which means they produce calming effects on the brain rather than stimulating effects.