What must be done when a flagger is required during a crossing?

When a flagger is required during a crossing, it is essential that the flagger signals traffic until the train clears the crossing. This ensures the safety of both the train and any vehicles or pedestrians approaching the crossing. The flagger's primary role is to control and direct traffic in order to prevent accidents, which is particularly crucial when trains are approaching or present at the crossing.

By maintaining visual and physical control of the traffic, the flagger can alert drivers and pedestrians to the presence of an oncoming train, allowing them to safely halt their movements until it is safe to proceed. This procedure not only enhances safety but also ensures compliance with operational standards meant to protect lives and property at railroad crossings.

The other options would not provide adequate safety measures or responsibility in this critical situation, which emphasizes the importance of the flagger’s role in keeping the area clear until the train has completely passed.