Which feature indicates that your vehicle is equipped with antilock brakes?

The feature that indicates your vehicle is equipped with antilock brakes (ABS) is the illumination of the ABS light when starting the vehicle. This light serves as a notification to the driver that the ABS system is present in the vehicle. During the vehicle's initial startup, the ABS light will briefly illuminate as part of a self-check process. If the light remains on after starting, it can indicate a malfunction in the ABS system, which would need further investigation.

The other options describe different scenarios related to braking but do not specifically confirm the presence of antilock brakes like the ABS light does. Changes in brake pedal feel may occur due to various conditions or systems but are not exclusively indicative of ABS. The service light turning on could relate to numerous vehicle maintenance or service needs, and horn activation during braking is not a standard feature associated with ABS systems. Therefore, the illumination of the ABS light at startup is the most direct indication of your vehicle's ABS feature.