While making moves under Rule 556, when may a dispatcher grant permission to pass a Stop Signal?

The correct response is rooted in the specific requirements of Rule 556, which governs the circumstances under which a dispatcher can permit a train to pass a Stop Signal. Granting permission to pass a Stop Signal is a serious action that is only permissible when the dispatcher has confirmed that the block to be entered is clear. This is crucial for ensuring the safety of railway operations; if the block is not clear, the train could conflict with other train movements or obstacles, potentially leading to serious accidents.

This rule emphasizes that safety is paramount and only specific conditions must be met before a dispatcher can provide such permission. The clarity of the block is a fundamental preventive measure to maintain safety on the rail network.

The other options, while they might seem reasonable, do not encapsulate the stringent requirements that Rule 556 imposes to ensure safety. Simply informing the train crew or declaring it 'safe' does not guarantee that the block is actually clear, and relying on cab signals alone does not take into account the overall situation on the tracks.