The correct answer is D. Flat mild steel bearing plates are used on all joints and curves, as well as for points and crossings in the lead portion.
Flat mild steel bearing plates are used to distribute the load from the rails to the sleepers. They are made of mild steel, which is a type of steel that is not as strong as other types of steel, but is more ductile. This means that it can be bent without breaking. Flat mild steel bearing plates are used on all joints and curves because they are able to withstand the stresses that are created at these locations. They are also used for points and crossings in the lead portion because they are able to withstand the high loads that are generated at these locations.
Option A is incorrect because flat mild steel bearing plates are not only used for points and crossings in the lead portion. They are also used on all joints and curves.
Option B is incorrect because flat mild steel bearing plates are not only used with wooden sleepers at locations where creep is likely to be developed. They are also used on all joints and curves.
Option C is incorrect because flat mild steel bearing plates are not only used on all joints and curves. They are also used for points and crossings in the lead portion.