Explanation: Radio waves travel at the speed of light. In a vacuum (and approximately in air), this speed is $3 \times 10^8$ meters per second. To calculate the time it takes for a radio wave to travel three nautical miles, we first convert the distance to meters. One nautical mile is defined as 1852 meters. So, 3 nautical miles = $3 \times 1852 \text{ meters} = 5556 \text{ meters}$. Now, using the formula Time = Distance / Speed: Time = $5556 \text{ meters} / (3 \times 10^8 \text{ meters/second})$ Time = $0.00001852 \text{ seconds}$ To express this in microseconds (where 1 microsecond = $10^{-6}$ seconds): Time = $0.00001852 \times 10^6 \text{ microseconds} = 18.52 \text{ microseconds}$. This value closely matches option D, 18.51 microseconds. Option A, 6.17 microseconds, is approximately the travel time for one nautical mile. The other options do not correspond to the correct calculation.