Which of the following substances is classified as a schedule 4 controlled substance?

The classification of controlled substances is determined by the potential for abuse, medical use, and safety or dependence liability. Schedule 4 substances are considered to have a lower potential for abuse relative to those in Schedule 1 through Schedule 3, and they are generally accepted for medical use in the United States.

Alprazolam, the correct answer, is classified as a Schedule 4 controlled substance. It is a benzodiazepine that is used primarily for the management of anxiety and panic disorders. The drug has a recognized medical use and is prescribed commonly, but it also carries a risk of dependence and abuse, which justifies its placement in Schedule 4.

In contrast, the other substances listed fall under different schedules. Diazepam, while also a benzodiazepine and also classified as Schedule 4, is not the focus here since the question specifies one choice. Oxycodone and dihydrocodeine, on the other hand, are classified as Schedule 2 substances due to their higher potential for abuse and risk of severe psychological or physical dependence. This classification reflects the need for stricter controls and regulations concerning their prescription and distribution.