Why This is the Nyquist sampling theorem: to reconstruct a signal without ambiguity, the sampling rate must be at least twice the highest frequency present in that signal. So a 3 kHz audio signal needs at least 6 kHz of sampling, and CD audio uses 44.1 kHz to cover hearing out to about 20 kHz. If you sample slower than that, higher frequency components fold back down into the passband as false lower frequencies, an error called aliasing, which is why an anti-alias lowpass filter is placed ahead of the ADC.
Watch out Sampling at the same rate as, or half the rate of, the highest frequency loses the information entirely and produces aliases; four times the highest frequency does work fine in practice but it is more than the theorem requires, and the question asks for the minimum.
Nyquist: sample at 2x the highest frequency. Twice, not once, not four times.