Which of the following duties must a dental assistant perform under direct supervision?

The duty of placing periodontal dressing must be performed under direct supervision because it involves specific skills and understanding of the periodontal tissue that are critical for ensuring patient safety and treatment efficacy. The task requires the dental assistant to work closely with the dentist to ensure that the dressing is applied effectively and correctly, complying with clinical guidelines and monitoring for any potential complications during the procedure.

In contrast, tasks such as mixing dental materials or taking patient medical histories may be completed under general supervision, as they do not directly manipulate a patient's oral health to the same degree. Applying cavity liners, while more involved than mixing materials, still falls under a different level of supervision due to variations in state regulations and specific dental practice frameworks that allow for certain tasks to be performed without the dentist being present in the immediate treatment area.