To determine which option correctly describes the function of a speedometer in a vehicle, we need to consider what a speedometer does in practice.

1. Understanding a Speedometer: A speedometer is an instrument found in vehicles that shows the current speed at which the vehicle is traveling. This is usually displayed in units such as kilometers per hour (km/h) or miles per hour (mph).
2. Examine the Options:
   1. It displays the current speed at which the vehicle is moving.
   2. It measures the total distance the vehicle has traveled.
   3. It calculates the average speed over a journey.
   4. It shows the rate at which the vehicle’s speed changes.
3. Analysis:
   • The first option correctly describes what a speedometer does—it tells the driver the vehicle's current speed, allowing them to adjust driving to comply with speed limits and drive safely.
   • The second option refers to an "odometer," a different instrument that measures the total distance traveled by the vehicle.
   • The third option describes an average speed, which is not displayed by a standard speedometer. Calculating average speed would require additional data processing beyond the function of a regular speedometer.
   • The fourth option might describe an accelerometer's function, as speedometers do not typically indicate the rate of speed change directly.
4. Conclusion: Therefore, the correct answer is: "It displays the current speed at which the vehicle is moving."

By understanding the basic function of a speedometer, we can accurately identify the correct option. This is essential knowledge in Basic Science and Engineering subjects, especially for vehicle dynamics and instrumentation.