What must be done when coming upon a red flag displayed between the rails ahead?

Answer: B — Stop and do not proceed until the flag has been removed by authorized personnel.

When encountering a red flag displayed between the rails, the appropriate action is to stop and not proceed until the flag has been removed by authorized personnel. A red flag serves as a critical safety signal indicating a hazardous situation or obstruction on or near the tracks. It is essential for the safety of both crew and equipment to fully assess the situation before moving forward. By stopping, the crew can ensure they await further instructions and confirmation that it is safe to continue, thereby minimizing the risk of accidents or incidents. Other choices may misinterpret the seriousness of a red flag. For instance, removing the flag without authority undermines safety protocols, proceeding cautiously without verification can still pose risks, and ignoring the flag completely disregards the established safety measures that are vital for safe train operation.