Which of the following is an example of a noncritical instrument in dental radiology?

The correct choice is film holding devices. In dental radiology, noncritical instruments are those that come into contact with intact skin but do not penetrate mucous membranes. Film holding devices are designed to hold radiographic films or digital sensors in place during exposure, coming into contact only with the patient's skin. As such, they are classified as noncritical instruments since they do not require sterilization, but they should be cleaned and disinfected after each use.

The other options are classified differently. The position indicating device (PID) is a part of the x-ray machine that directs the beam of radiation and does not touch the patient, but it needs to be maintained and cleaned regularly to ensure proper functioning and safety. Dental chairs are considered semi-critical items because they may come into contact with bodily fluids, requiring disinfection but not sterilization. Mirrors, while used in intraoral examinations, can also be considered semi-critical since they come into contact with mucous membranes and require sterilization or high-level disinfection after use.