Which of the following benzodiazepines is listed as a Schedule IV controlled substance?

Diazepam is classified as a Schedule IV controlled substance under the Controlled Substances Act. This classification indicates that it has a recognized medical use but also carries a risk for abuse and dependence, which necessitates regulation. Schedule IV substances are considered to have a lower potential for abuse relative to Schedule I-III substances, yet can still lead to psychological or physical dependence.

Benzodiazepines, such as diazepam, are commonly prescribed for anxiety, seizures, muscle spasms, and other medical conditions, making this classification appropriate. The regulation of diazepam as a Schedule IV substance reflects the need for careful monitoring of its prescription and dispensing.

In contrast, hydrocodone is classified as a Schedule II controlled substance, indicating a higher potential for abuse. Phenobarbital is generally classified as a Schedule IV substance in some contexts but may be considered a Schedule III substance in others, depending on its formulation and use. Diphenhydramine, while an antihistamine commonly used for allergies and sleep, is not a controlled substance and is available over-the-counter. This distinction helps clarify why diazepam stands out as the correct answer in this context.