What indicator shows that the trailer is equipped with antilock brakes?

The presence of an antilock braking system (ABS) on a trailer is indicated by a yellow ABS malfunction lamp. This lamp serves a critical function; when illuminated, it alerts the driver to any issues with the ABS system that may need addressing. The yellow color is a standard in signaling caution, making it clear that it’s important for the driver to investigate further to ensure safety and proper operation.

Other indicators, such as red lights, typically signal immediate issues or operational failures. Green lights are generally associated with normal operation conditions and not specifically designed for ABS warnings. Blue signals are not standard in the context of ABS indicators, so they would not provide relevant information concerning the system's functionality. Hence, the yellow ABS malfunction lamp is the proper indicator for a trailer equipped with antilock brakes.