Explanation: Radar range is determined by the time it takes for an electromagnetic wave to travel from the antenna to a target and back. The round-trip time for a signal to cover one nautical mile of range is approximately 12.36 microseconds. To generate range rings at intervals of 0.25 nautical miles, the marker oscillator must produce a pulse for every 0.25 nautical miles of round-trip travel time. 1. **Calculate the time period:** Time Period = 0.25 nm * 12.36 µs/nm = 3.09 microseconds. 2. **Calculate the frequency:** Frequency ($f$) is the reciprocal of the time period ($T$): $f = 1 / T = 1 / (3.09 \times 10^{-6} \text{ s}) \approx 323,624 \text{ Hz}$ This frequency is approximately 322 kHz. Options A, C, and D correspond to different range intervals (e.g., 161 kHz for 0.5 nm intervals).