If a red LED on the SR-3 relay board illuminates, what does it indicate?

When a red LED on the SR-3 relay board illuminates, it typically indicates an issue related to the coil of a relay. Specifically, the illumination of the red LED signifies that the relay coil has not been energized, which can occur due to reasons like an open circuit or a blown fuse.

In the context of the choices, a successful energization of the output circuit would generally be indicated by a different status, such as a green LED. The presence of full battery voltage is not directly indicated by the LED status but can be checked separately. Additionally, if the relay coil had been energized but the contacts failed to close, one would expect a different behavior from the LED.

Thus, while the correct choice identifies a situation where the relay coil is not energized, it clarifies the direct impact of the LED illumination in signaling relay operation faults rather than successful energization.