The correct answer is: A. First Stop signal.
A first stop signal is a railway signal that controls the entry of trains into the block section in advance. It is located at the entrance to the block section and is the first signal that a train will encounter when approaching the block section. The first stop signal is usually a semaphore signal, but it can also be a light signal or a track circuit.
A first stop signal is important because it ensures that trains do not enter a block section until it is safe to do so. This prevents collisions between trains and helps to ensure the safe operation of the railway.
The other options are incorrect because they do not accurately describe the function of a first stop signal. A stop signal is a railway signal that indicates that a train must stop before proceeding. A last stop signal is a railway signal that indicates that a train must stop before entering a station. None of these signals control the entry of trains into a block section in advance.