Which of the following describes an over-the-counter drug?

An over-the-counter drug is characterized by its availability to consumers without the requirement of a prescription from a veterinarian. This means that anyone can purchase the drug directly from a pharmacy or store, making it accessible for treating minor health issues or providing supportive care. Over-the-counter drugs are typically considered safe and effective for use by the general public, provided that the users follow the directions for use.

The other options refer to prescription medications, which necessitate approval and oversight from a licensed veterinarian to ensure the safe and appropriate use of the drug. These medications might be more potent or used for conditions requiring careful monitoring, which is why they are not available over-the-counter. Thus, the defining feature of over-the-counter drugs is their unrestricted availability to the general public without professional oversight.