Final answer:
The pulse with a PRF of 20 kHz has the shortest pulse repetition period.
Step-by-step explanation:
The pulse repetition period (PRP) is the time it takes for a pulse to repeat itself. It is the reciprocal of the pulse repetition frequency (PRF). To find the shortest pulse repetition period, we need to find the largest PRF.
In this case, the PRF values are: A) 12 kHz, B) 6,000 Hz, C) 20 kHz, D) 1 kHz.
The largest PRF is 20 kHz, so option C has the shortest pulse repetition period.