The desirable rate of change of cant deficiency in case of Metre Gauge is A. 20 mm/sec B. 35 mm/sec C. 55 mm/sec D. 65 mm/sec

The correct answer is A. 20 mm/sec.

Cant deficiency is the difference between the cant of the track and the cant required for the design speed of the train. The desirable rate of change of cant deficiency is 20 mm/sec for metre gauge. This means that the cant deficiency should change by 20 mm in every second. This is to ensure that the train does not experience any sudden changes in the cant, which could cause it to derail.

Option B is incorrect because it is too high. A rate of change of cant deficiency of 35 mm/sec would be too much for the train to handle and could cause it to derail.

Option C is incorrect because it is too low. A rate of change of cant deficiency of 55 mm/sec would not be enough to ensure that the train does not experience any sudden changes in the cant.

Option D is incorrect because it is not a valid option.