Which of the following substances is classified under Schedule II due to its potential for addiction and abuse?

Secobarbital is classified as a Schedule II controlled substance primarily due to its high potential for addiction and abuse. Schedule II substances are those that have a recognized medical use but also carry a significant risk of dependency, both psychological and physical. Secobarbital, a barbiturate, is prescribed for short-term treatment of insomnia and as a pre-anesthetic. However, it has a high likelihood of misuse, leading to its placement under Schedule II.

On the other hand, while alprazolam and clonidine are prescribed medications, alprazolam is categorized as a Schedule IV substance, indicating a lower potential for abuse compared to Schedule II drugs. Clonidine, primarily used for hypertension and ADHD, is not classified as a controlled substance at all. Codeine, while it can be a Schedule II substance when in its pure form for severe pain management, is often found in lower concentrations within certain formulations (e.g., cough syrups) that might be classified as Schedule III. Therefore, the classification of secobarbital as a Schedule II substance makes it the correct answer here based on its potential for addiction and abuse.