What is true about the frequency of a radio signal?

The correct choice emphasizes the impact of the Doppler effect on the frequency of a radio signal when there is relative motion between the radar and the target. When a target is moving towards the radar, the frequency of the reflected signal increases, while it decreases if the target is moving away. This change in frequency is a crucial aspect of radar operations as it allows for the determination of the target's speed and direction.

Understanding how the frequency can vary in relation to the movement of targets is essential for operators to accurately interpret radar data and perform effective monitoring and tracking of objects. This principle underpins many radar applications, such as law enforcement speed detection and weather radar analysis.