When referring to an anti-lock air brake system, which of the following would be an actuator?

In an anti-lock air brake system, the actuator is a component that physically engages or controls the brakes in response to signals from the system. The brake modulator valve plays a critical role in this process. It regulates the air pressure to the brake chambers, allowing for precise control over the braking force applied to the wheels. This modulation of brake pressure is essential for preventing wheel lockup during braking, which is one of the primary functions of an anti-lock system. By adjusting the air pressure dynamically based on the vehicle's speed and traction conditions, the brake modulator valve ensures optimal braking performance and safety.

Other components listed, such as brake lights, axle tone rings, and wheel speed sensors, serve different functions. Brake lights are indicators for other drivers and do not participate in the braking process. The axle tone ring and wheel speed sensors work together to detect wheel rotation and speed but do not directly control the brake application. These sensors provide critical data to the anti-lock system, but they are not considered actuators themselves.