Final answer:
A person addicted to crystal meth is using a stimulant, which is a type of drug that increases alertness and neural activity.
Step-by-step explanation:
A person addicted to crystal meth is using a stimulant. Methamphetamine, commonly known as crystal meth, is a highly addictive stimulant that increases levels of neural activity and acts as an agonist of the dopamine neurotransmitter system. This results in feelings of euphoria, increased energy, and alertness. However, its use has significant adverse consequences including potential for addiction, health issues, and alteration of brain functioning. Stimulants like methamphetamine are compared to depressants, which have the opposite effect by calming the brain and reducing arousal. The best answer to the question is: A. stimulant.