The correct answer is: C. SOR
A corridor block is a type of railway block system in which a train is allowed to proceed into a section of track only if the block ahead is clear. This is done by using a system of signals and track circuits to monitor the occupancy of the track.
The SOR (Signalling and Operating Rules) is a document that sets out the rules for the safe operation of railways in the United Kingdom. It includes a section on corridor block working, which describes the procedures that must be followed when using this type of block system.
The SWR (Safety and Working Rules) is a similar document that sets out the rules for the safe operation of railways in Australia. It does not include a section on corridor block working, as this type of block system is not used in Australia.
The WTT (Working Timetable) is a document that sets out the timetable for train services on a railway network. It does not include any information on the safe operation of railways, as this is covered by the SOR or SWR.