What signifies that a block is considered clear in terms of signaling?

A block is considered clear when a green signal is displayed. In rail operations, signals communicate the status of the track ahead to the train crews. A green signal specifically indicates that the block is clear for a train to proceed safely without restrictions. This is a critical part of maintaining safe and efficient train movements, ensuring that crews can trust that the pathway ahead is free of obstructions or other trains.

The other options involve forms of communication or verification that do not independently indicate the status of the block. While verification by the conductor, notification by the dispatcher, or confirmation from other trains can play important roles in operational safety and awareness, they do not serve as the definitive signal that a block is clear. Only a green signal provides the immediate, visual indication required by crews to proceed confidently.