Final answer:
An agent would need to consent for the 21-year-old man in a coma, as he is unable to provide informed consent himself. A healthcare proxy or similar legal document would authorize this agent to make medical decisions.
Step-by-step explanation:
The situation legally requiring an agent to consent to medical treatment on behalf of the patient is D. A 21-year-old man in a coma. In this scenario, the patient is incapacitated and unable to provide informed consent due to his medical condition. As such, a healthcare proxy or legal document may appoint a specific person, such as a family member or designated agent, to make medical decisions on the patient's behalf. It's important to understand that different states may have varying laws regarding who can serve as a healthcare proxy and the extent of their decision-making powers. In cases involving minors, pregnant females, or specific legal circumstances, additional factors and state laws would need to be considered for informed consent.