Which of the following factors cannot reduce patient exposure and operator exposure?

The choice indicating that keeping patient-screen distance to a maximum cannot reduce patient and operator exposure is accurate because increasing the distance between the patient and the imaging screen does not help minimize exposure. In radiation physics, increased distance typically results in reduced exposure; however, maximizing patient-screen distance actually includes the risk of increased exposure to scatter radiation.

In contrast, the other options focus on effective strategies to minimize exposure. Restricting x-ray beam "on" time to a minimum minimizes the duration of radiation exposure. Using the lowest milliampere (mA) and highest kilovolt peak (kVp) settings maximizes image quality while minimizing dose, as higher kVp can result in greater penetration and lower necessary mA. Lastly, restricting the x-ray beam to the smallest size reduces the volume of tissue exposed to radiation, effectively lowering the overall exposure for the patient and any nearby operators.