The correct answer is A. 200 meters.
Block overlap is the distance between the front of a train and the rear of the train in front of it. In a multiple aspect signalling system, block overlap is used to prevent trains from colliding. The system uses a series of signals to indicate whether a track is clear or occupied. When a train approaches a signal, it will stop if the signal is red. If the signal is green, the train can proceed.
The amount of block overlap is determined by the speed of the trains and the distance between the signals. In general, the faster the trains, the greater the need for block overlap. The distance between the signals is also important, as a longer distance between signals will require a greater amount of block overlap.
In the United States, the standard block overlap for a multiple aspect signalling system is 200 meters. This means that a train must stop if it is within 200 meters of the rear of the train in front of it. This helps to ensure that there is always enough space between trains to prevent collisions.
The other options are incorrect because they do not represent the standard block overlap for a multiple aspect signalling system in the United States.