Which of the following is an example of a controlled substance?

Opium is classified as a controlled substance due to its potential for abuse and dependence. Controlled substances are drugs that are regulated by law because they possess a high potential for addiction or misuse. Opium falls under the category of narcotic drugs derived from opium poppy, and it is subject to strict legal controls regarding its manufacture, distribution, and use.

The other options, while they may have therapeutic uses, do not fall under the same level of regulatory control as opium. Ibuprofen and acetaminophen are over-the-counter medications used for pain relief and do not have the same abuse potential as controlled substances. Penicillin, an antibiotic, is also not classified as a controlled substance and is simply monitored for safety and efficacy against bacterial infections. Thus, the classification of opium as a controlled substance is based on the need to manage the risk associated with its use in medical and recreational settings.