The correct answer is option 'A', a radar speedometer is used to measure spot speed. Let's understand the concept in detail:


A radar speedometer is a device that uses radar technology to measure the speed of a moving vehicle. It uses the Doppler effect principle to determine the speed of an object by measuring the change in frequency of the radar waves reflected from the object.


Spot speed refers to the instantaneous speed of a vehicle at a specific point in time. It is the speed of a vehicle at a particular location, measured at a specific moment.


Radar speedometers are commonly used by traffic police and law enforcement agencies to measure the spot speed of vehicles. They are usually mounted on a tripod or a stationary vehicle and aimed towards the road. The radar waves emitted by the speedometer are reflected by passing vehicles, and the device calculates the speed of the vehicle based on the change in frequency of the reflected waves.


Radar speedometers have several advantages over other speed measurement methods:

- Accuracy: Radar speedometers provide accurate measurements of spot speed, allowing law enforcement agencies to enforce speed limits effectively.
- Non-intrusive: These speedometers can measure the speed of multiple vehicles without requiring them to stop or slow down.
- Real-time data: Radar speedometers provide real-time data, allowing immediate action to be taken if a vehicle is found to be exceeding the speed limit.
- Versatility: They can be used in various conditions, including day and night, and in different weather conditions.
- Remote operation: Some radar speedometers can be operated remotely, allowing law enforcement officers to monitor speed without being present at the location.


In conclusion, a radar speedometer is primarily used to measure spot speed, providing accurate and real-time data on the instantaneous speed of a vehicle at a specific location. This information is crucial for enforcing speed limits and ensuring road safety.