The correct answer is A. 1 in 360.
Cant gradient is the maximum permissible slope of a railway track. It is the ratio of the change in elevation to the horizontal distance over which the change occurs. The limiting value of cant gradient is 1 in 360, which means that for every 360 feet of horizontal distance, the track can rise or fall by 1 foot. This is the maximum slope that can be safely negotiated by trains.
Option B, 1 in 720, is not a valid cant gradient. Option C, 1 in 1000, is a valid cant gradient, but it is not the limiting value. Option D, 1 in 1200, is also a valid cant gradient, but it is not the limiting value.
The limiting value of cant gradient is 1 in 360 because this is the maximum slope that can be safely negotiated by trains. If the slope is too steep, the train may derail. The limiting value of cant gradient is also determined by the type of track. For example, high-speed tracks have a lower limiting value of cant gradient than low-speed tracks.