Final answer:
In digital radiography, Nyquist refers to the sampling frequency of the image, which must be at least twice the highest frequency present in the signal to accurately reconstruct it.
Step-by-step explanation:
In digital radiography, Nyquist is referring to c. The sampling frequency of the image. The Nyquist theorem, or Nyquist criterion, is named after engineer Harry Nyquist. This principle pertains to the sampling rate required for a signal. It states that to accurately reconstruct a signal, it must be sampled at least at twice the highest frequency present in the signal. In the context of digital radiography, this relates to the number of times that the radiographic image is sampled and digitized. As the sampling frequency increases, so does the ability to define the image's detail. But if the sampling frequency is too low, it will result in aliasing, where the image may appear distorted or may not accurately represent the original signal.