When can the hand brakes be released once the engine is coupled to a train or cars standing on a grade?

The correct answer is that the hand brakes can be released once the air brakes are fully charged. This is a critical safety procedure in railroad operations. When an engine is coupled to a train or cars that are standing on a grade, it is essential to ensure that the air brake system is fully operational before releasing the hand brakes. If the air brakes aren't fully charged, there is a risk that the train could roll away due to gravity.

By waiting until the air brakes are fully charged, the crew ensures that they can effectively control the train's movement. This step is vital for maintaining safety and preventing accidents associated with uncontrolled movement. Proper air brake functionality allows for adequate stopping power and control, which is especially important when managing a train on a grade.

Other options may seem plausible but do not align with safety protocols in rail operations. For instance, visually confirming might not provide an assurance of the air brake's readiness, and simply waiting for a minute doesn't guarantee that the air brakes have charged. Releasing the hand brakes only when the train starts moving would be counterproductive and could lead to an unsafe situation before the train is adequately secured.