Which of the following is a duty that an RDA can perform under direct supervision?

The option indicating the duty that an RDA can perform under direct supervision is the correct choice. Under California Dental Law, certain procedures can only be carried out by a Registered Dental Assistant (RDA) when a licensed dentist is present and overseeing the practice. The act of testing pulp vitality is a procedure that requires the definitive judgment of a licensed dentist, and thus falls under the category of tasks that must be conducted during direct supervision.

In contrast, polishing and contouring existing amalgam restorations, placing a rubber dam, and fabricating indirect restorations are tasks that are more technical in nature and can be performed by an RDA with varying degrees of supervision. Polishing amalgam restorations and placing a rubber dam may not require direct supervision at all and can be performed under general supervision, depending on specific circumstances and training. Fabricating indirect restorations typically requires more advanced skills and knowledge, which may not be authorized under the direct supervision of a licensed dentist for an RDA.