Final answer:
Dependency is best defined by a change in the nervous system that results in needing the drug to prevent withdrawal symptoms. It is related to physical and psychological dependence, tolerance, and addiction.
Step-by-step explanation:
The statement that best defines dependency is: C) A change in the nervous system occurs so that a person now needs to take the drug to prevent withdrawal symptoms. This condition arises when users of a psychoactive drug develop a physical or psychological reliance on it, wherein stopping its use leads to withdrawal symptoms. Physical dependence involves changes in normal bodily functions that cause withdrawal symptoms upon cessation of drug use. Tolerance is a related concept where a user requires higher doses of the drug to achieve effects once experienced at lower doses, which is a manifestation of physiological dependence. Psychological dependence refers to the emotional need for a drug, often to relieve psychological distress or because of drug craving. Addiction encompasses both physical and psychological aspects and involves the compulsive use of a substance despite its adverse consequences.