When is it crucial for the train operator to stop the train according to the specified procedures?

The correct response highlights an essential protocol in train operations regarding the observation of a white light. In rail systems, a white light often signifies specific instructions or conditions that require adherence to safety protocols. Typically, it can indicate a "clear" signal or other safety-related communications from the control center, necessitating careful attention from the train operator.

Stopping the train upon seeing a white light aligns with ensuring passenger safety and compliance with operational standards. Operators are trained to interpret signals accurately, and this signal's presence is a direct indication that action must be taken, either to stop and assess the situation or to prepare for further instructions.

This choice underscores the importance of following established safety measures and signals, which are crucial in preventing accidents and ensuring the safe operation of the train system.