The correct answer is: C. Yellow
When a front portion of a parted train gives one long one short one long one short whistle, the signalman (SM) should wave a yellow signal four times above their head from side to side and acknowledge. This is to indicate that the train is approaching and that the SM is ready to receive it.
A red signal indicates that the train should stop. A green signal indicates that the train can proceed. A yellow signal indicates that the train should proceed with caution.
The SM should wave the yellow signal four times above their head from side to side to ensure that the train driver sees it. The SM should also acknowledge the train’s arrival by waving the yellow signal back. This is to confirm that the train has been received and that the SM is ready to proceed with the next movement.