Which of the following is an example of a chemotherapy agent?

Chemotherapy agents are specifically designed to kill or inhibit the growth of cancer cells, utilizing various mechanisms to disrupt their ability to proliferate. In this context, platinum agents such as cisplatin serve as a quintessential example of chemotherapy. Cisplatin works primarily by causing cross-linking of DNA, which ultimately hinders replication and leads to cell death, making it effective in treating various types of tumors, including testicular, ovarian, and lung cancers.

The other options represent different categories of cancer treatment. Hormonal agents, while they are used in cancer therapy, typically modulate hormone levels or block hormone receptors and are not classified as traditional chemotherapy. Biologic agents harness the body's immune system or use molecules that target specific characteristics of cancer cells rather than poisoning them directly, which differs fundamentally from the action of conventional chemotherapy agents. Watchful waiting is a management strategy involving close monitoring of a patient's condition without immediate intervention and is not a treatment modality in the context of chemotherapy.