The correct answer is: B. Catch siding
A catch siding is a type of siding that is used to stop trains from derailing on a rising gradient. It is typically located at the top of a hill, and is designed to catch the train if it starts to derail. Catch sidings are usually made of concrete or steel, and are sloped so that the train will come to a stop without derailing.
A long siding is a type of siding that is used to store trains. It is typically located away from the main track, and is long enough to accommodate several trains. Long sidings are usually made of gravel or asphalt, and are often fenced in to prevent unauthorized access.
A slip siding is a type of siding that is used to allow trains to switch tracks. It is typically located at the intersection of two tracks, and is designed to allow the train to move from one track to the other without derailing. Slip sidings are usually made of concrete or steel, and are sloped so that the train will move smoothly from one track to the other.
None of these is the correct answer because it does not describe a type of siding that is used in case of 1 in 100 rising gradient.