Final answer:
When the pulse repetition frequency (PRF) is increased and the pulse duration remains constant, the duty factor (DF) increases because the pulse occupies a larger proportion of each cycle.
Step-by-step explanation:
If the system increases the pulse repetition frequency (PRF), the duty factor (DF) will increase. Duty factor is defined as the ratio of the pulse duration to the period of the pulse (which is inversely proportional to the PRF). Thus, when PRF increases, the period decreases, and assuming pulse duration remains constant, the DF increases because the pulse occupies a larger proportion of each cycle.