What does an outer home signal protect?

The outer home signal is specifically designed to protect a neutral zone outside the home signal. This means that when a train approaches an outer home signal, it is guided to ensure that it does not enter into a potentially unsafe situation at the home signal ahead. The neutral zone is a crucial area where trains can safely stop or be held while waiting for proper authorization to proceed. By protecting this zone, the outer home signal plays an essential role in maintaining the safe operation of train movements and preventing collisions or accidents.

The other options do not accurately represent the function of the outer home signal. It doesn't protect the entire railway line as this would imply a broader scope than its specific operational function. While it does contribute to the overall safety of the railway system, it does not protect the yard directly or protect the block ahead, which are functions assigned to other signaling systems and signals in the network.