Which of the following limits the highest frequency signal that can be accurately displayed on a digital oscilloscope?
Why A digital scope works by taking discrete samples of the waveform and reconstructing it, so the analog-to-digital converter's sampling rate sets the ceiling on what it can faithfully show. The Nyquist criterion says you need at least two samples per cycle just to detect a frequency, and in practice scope makers want roughly 5 to 10 samples per cycle for a usable waveform picture. So a scope sampling at, say, 1 gigasample per second cannot honestly display signals anywhere near that rate; undersampling produces aliasing, a false low-frequency image of the real signal.
Watch out The idea of an 'analog-to-digital converter reference frequency' sounds plausible but is not the limiting spec; ADCs have a voltage reference that sets amplitude scaling, not a frequency reference. Q describes the sharpness of a resonant circuit and has nothing to do with scope bandwidth.
Digital scope = samples. Nyquist: you need at least 2 samples per cycle, so sample rate caps the top frequency.