The correct answer is C. G & SR.
The Ruling Gradient of a section is the maximum gradient that a train can safely travel on. It is determined by the weight of the train, the type of track, and the condition of the track. The Ruling Gradient is usually found in the G & SR, which is a document that contains information about the track and its condition.
SWR stands for Standing Wave Ratio. It is a measure of the mismatch between a transmitter and its load. A high SWR can cause damage to the transmitter and the load.
WTT stands for Wireless Telegraphy Test. It is a test that is used to determine the quality of a wireless telegraphy system.
Operating Manual is a document that contains instructions on how to operate a piece of equipment. It is not a reliable source of information about the Ruling Gradient of a section.