What limitation applies when performing maintenance under a roadway maintenance machine?

The correct choice highlights a critical safety protocol that helps ensure the safety of employees while performing maintenance under a roadway maintenance machine. Specifically, no part of the employee should break the plane of the rail of the occupied track towards the adjacent controlled track. This rule is in place to protect workers from potential hazards associated with moving trains or equipment on the tracks. By preventing any part of the employee's body from crossing this boundary, it minimizes the risk of injury or accidents that could occur if an unexpected movement happens on the adjacent track while maintenance work is underway.

This adherence to safety standards is paramount, especially in environments where multiple tracks are in use, as it helps maintain a clear zone around live tracks, reinforcing the importance of vigilance and caution in maintenance operations. The other options do not prioritize such safety concerns, therefore they do not reflect the necessary precautions one must take when working in the context of railway maintenance.