Which response is necessary when the engineer receives permission to pass a restricted signal?

When an engineer receives permission to pass a restricted signal, the necessary response is to stop the front end of the head car in line with the signal. This action is crucial for ensuring safety when approaching a restricted signal. Stopping allows the engineer to assess the situation further, determine any potential hazards, and ensure that it is safe to proceed.

Stopping the train provides an opportunity to look out for any track conditions, potential obstructions, or any other trains in the area. Additionally, stopping in line with the signal ensures that everyone on board is aware of the status of the operation and can proceed with caution.

The other responses do not prioritize safety in a manner that aligns with standard operating procedures for passing a restricted signal, which makes stopping the head car in line with the signal the most responsible choice.