Among adolescents the most serious drug problem ismarijuana.cocaine.tobacco.alcohol.

To determine which substance presents the most serious drug problem among adolescents, let's analyze each option based on common findings.

1. Marijuana: While its use has become more socially accepted in recent years, marijuana can still lead to dependency and has potential negative effects on mental health, especially for adolescents.

2. Cocaine: Cocaine is a powerful stimulant and its use can lead to severe physical and psychological health issues. However, its prevalence among adolescents is generally lower compared to other substances.

3. Tobacco: Tobacco smoking rates have been declining, but it still poses significant long-term health risks including addiction, despite being less popular among younger individuals nowadays than in previous decades.

4. Alcohol: Alcohol is often the most accessible and commonly used substance among adolescents. Its consumption can lead to risky behaviors, addiction, and various negative health outcomes.

Taking all factors into consideration, alcohol tends to present the most serious drug problem among adolescents, primarily due to its high prevalence, immediate availability, and significant impact on health and behavior.