Gateway theory posits that initial use of alcohol, tobacco, or marijuana increases the likelihood of using more addictive drugs.

• A. Adjunct
• B. Designer
• C. Illicit
• D. Gateway

Answer: (D)

Gateway theory describes a progression in which using alcohol, tobacco, or cannabis increases the likelihood of trying more addictive or harder drugs. The idea is that early exposure can alter risk factors, social networks, and access, making later use of stronger substances more probable. Among the options, the term that precisely captures this progression is gateway. The other terms refer to different ideas: adjunct means something added to support another thing, designer refers to newly created synthetic drugs, and illicit describes drugs that are illegal in general. So gateway best fits the notion of a sequence from initial, common substances to later, more dangerous drugs.