The correct answer is A. 40 mm.
Cant deficiency is the difference between the cant of the track and the cant deficiency allowance. Cant is the sideways tilt of the track, which helps to keep trains on the track. The cant deficiency allowance is the maximum amount of cant deficiency that is allowed.
The cant deficiency allowance is different for different types of track. For meter gauge routes, the cant deficiency allowance is 40 mm. This means that the maximum amount of cant deficiency that is allowed on a meter gauge route is 40 mm.
If the cant deficiency is greater than the cant deficiency allowance, it can cause the train to derail. This is why it is important to keep the cant deficiency within the allowed limits.
The other options are incorrect because they are not the maximum amount of cant deficiency that is allowed for meter gauge routes.