The correct answer is A. T-509.
T-509 is a railroad signal that indicates that a train must stop before proceeding. It is a red light that is displayed at the approach to a crossing. When a T-509 signal is displayed, trains must come to a complete stop and wait for the signal to change to green before proceeding.
B. T-369(3b) is a railroad signal that indicates that a train must proceed with caution. It is a yellow light that is displayed at the approach to a crossing. When a T-369(3b) signal is displayed, trains must slow down and be prepared to stop if necessary.
C. T-806 is a railroad signal that indicates that a train can proceed through a crossing without stopping. It is a green light that is displayed at the approach to a crossing. When a T-806 signal is displayed, trains can proceed through the crossing at normal speed.
D. A simple memo is a written communication that is used to convey information or instructions. It is typically used to communicate with employees or other members of an organization. A simple memo would not be used to indicate that a stop signal is to be passed in On position.