Trailers receive instructions as to when to apply and release the brakes from...?

Answer: C — The tractor

Trailers receive instructions regarding when to apply and release the brakes primarily from the tractor. This is because the braking system in a combination of a tractor and trailer is designed to work together for optimal safety and efficiency. The tractor communicates with the trailer's braking system, ensuring that when the driver applies the brakes in the tractor, the trailer responds accordingly. This synchronous braking is crucial for maintaining control during braking maneuvers, especially in a fully loaded or combined vehicle scenario. The driver has control over the tractor's braking system but relies on the trailer's braking system to respond to their commands. Additionally, while modern trailers may have onboard computer systems that provide diagnostic feedback or assist with various functions, the fundamental control for applying and releasing the brakes remains linked to the tractor itself. This is essential for coordinated stopping power, preventing issues like jackknifing and improving overall road safety.