Final answer:
Increasing the Pulse Repetition Period (PRP) leads to a decrease in the duty factor, which is the fraction of time the ultrasound system is actively transmitting pulses.
Step-by-step explanation:
When the Pulse Repetition Period (PRP) increases, the amount of time between ultrasound pulses extends, which results in a decrease in the number of pulses that occur in a given time period. Since the duty factor is defined as the fraction of time that the system is actively transmitting pulses, an increase in PRP means that the system is active for a smaller fraction of the time. Therefore, when the PRP increases, the duty factor decreases.