Which of the following drugs is classified as a Schedule II controlled substance?

Cocaine is classified as a Schedule II controlled substance due to its potential for abuse and the possibility of causing severe psychological or physical dependence. Schedule II medications are highly regulated; they can be prescribed but require strict adherence to legal requirements for their dispensation.

In contrast, fluoxetine, amoxicillin, and diphenhydramine fall under different categories. Fluoxetine is an antidepressant classified as a selective serotonin reuptake inhibitor (SSRI) and is not considered controlled due to its lower potential for abuse. Amoxicillin is a penicillin antibiotic that is also not controlled, as it does not have a significant risk of abuse or dependence. Diphenhydramine is an antihistamine used to treat allergies and is not a controlled substance either, as it does not present a risk of abuse. Thus, cocaine stands out as the only drug among the choices that is classified as a Schedule II controlled substance due to its defined risks and regulations surrounding its use.