The correct answer is C. 25 cm.
The minimum depth of ballast cushion for a Broad Gauge wooden sleeper of size 275x25x13 cm with 75cm sleeper spacing is 25 cm. This is to ensure that the sleeper is properly supported and that the ballast is able to provide adequate drainage.
The ballast cushion is a layer of material that is placed between the sleeper and the subgrade. It serves to distribute the load from the sleeper evenly over the subgrade, and to provide drainage. The ballast cushion is typically made of crushed stone or gravel, and it should be at least 25 cm thick for a Broad Gauge wooden sleeper with 75cm sleeper spacing.
If the ballast cushion is too thin, the sleeper will not be properly supported and it may settle. This can lead to uneven track, and it can also damage the sleeper. If the ballast cushion is too thick, it will not be able to provide adequate drainage. This can lead to waterlogging of the subgrade, which can cause the track to become unstable.
Therefore, it is important to ensure that the ballast cushion is the correct thickness. The minimum depth of ballast cushion for a Broad Gauge wooden sleeper of size 275x25x13 cm with 75cm sleeper spacing is 25 cm.