Explanation: Pulse Repetition Frequency (PRF) and Pulse Repetition Interval (PRI) are inversely related. PRF is the number of pulses transmitted per second, while PRI is the time duration between the start of one pulse and the start of the next. To find the PRI, you divide 1 by the PRF: PRI = 1 / PRF Given PRF = 2500 Hz: PRI = 1 / 2500 Hz = 0.0004 seconds To convert seconds to microseconds (µs), multiply by 1,000,000 (since 1 second = 1,000,000 µs): PRI = 0.0004 seconds * 1,000,000 µs/second = 400 µs Therefore, the PRI is 400 microseconds. The other options are incorrect as they do not represent the inverse of a 2500 Hz PRF.