Answer:
40 KHz
Explanation:
The minimum sampling frequency is at least twice the highest frequency component in the analog signal. In this case the highest frequency component is 20 KHz. Hence the minimum sampling frequency will be 2*20 KHz = 40 KHz. This minimum sampling frequency is also known as Nyquist frequency. If we sample at less than the Nyquist frequency, the original signal cannot be reconstructed from the samples.
