The correct answer is C. On memo.
A PLCT (Power Line Clearance Ticket) is a document that authorizes the clearing of trees and other obstructions from power lines. A T/A602 (Train Authorization) is a document that authorizes the movement of a train on a particular track. A memo is a written communication that is used to convey information or instructions.
In the case of an obstructed block section, the relief train will be dispatched on the authority of a memo from the train dispatcher. The memo will authorize the movement of the relief train and will specify the route that the train should take.
The other options are incorrect because they do not authorize the movement of a relief train. A PLCT authorizes the clearing of trees and other obstructions from power lines, but it does not authorize the movement of a train. A T/A602 authorizes the movement of a train on a particular track, but it does not authorize the movement of a train on an obstructed block section.