Which instrument displays the airspeed in the Cessna 172?

The airspeed indicator is the primary instrument used to display the aircraft's airspeed in the Cessna 172. It functions by measuring the dynamic pressure of the air flowing into the aircraft and comparing it to a reference pressure from a static source. This pressure difference is then displayed on the instrument in knots or miles per hour, depending on the design of the aircraft's gauge.

Understanding airspeed is crucial for pilots because it affects various aspects of flight, such as climb performance, stall speeds, and operating limits. The airspeed indicator is typically marked with color-coded bands that indicate the different phases of flight, including normal operating range, cautionary speeds, and stall warnings.

Other instruments mentioned in this question have different functions: the altimeter measures altitude based on atmospheric pressure, the vertical speed indicator indicates the rate of climb or descent, and the heading indicator provides information on the aircraft's magnetic heading. None of these instruments provide information about the airspeed, which is why the airspeed indicator is the correct choice.