Explanation: Range markers, typically used in radar systems, indicate the one-way distance to a target. This distance is determined by the time it takes for a radio wave to travel from the source to the target and back (round trip). 1. **Calculate the period (T) of the controlling oscillator:** The period is the inverse of the frequency. $T = 1 / \text{Frequency} = 1 / 20,000 \text{ Hz} = 0.00005 \text{ seconds} = 50 \mu\text{s}$. 2. **Relate period to distance:** Radio waves travel at the speed of light. A common approximation for radar calculations is that radio waves travel approximately 1 nautical mile in $12.36 \mu\text{s}$ (one-way trip) or $6.18 \mu\text{s}$ (for a round trip of 1 NM total distance). 3. **Calculate the distance:** Since the oscillator's period of $50 \mu\text{s}$ corresponds to the time between markers (representing a round-trip delay), we use the one-way travel time equivalent: Distance = $50 \mu\text{s} / (12.36 \mu\text{s/NM}) \approx 4.045 \text{ nautical miles}$. Therefore, the distance between range markers is approximately 4 nautical miles. Option D (8 nautical miles) would represent the total round-trip distance covered in one period, not the one-way range indicated by the marker. Options A and B are too small for this frequency.