The correct answer is: B. 3200 mm
The maximum width of coaching stock in BG is 3200 mm. This is the standard width of all Indian Railways coaches, and it is important to ensure that all coaches are within this width to avoid any problems when they are being transported or when they are being coupled together.
The other options are incorrect because they are not the standard width of Indian Railways coaches. Option A is 3050 mm, which is too narrow. Option C is 3250 mm, which is too wide. Option D is 3300 mm, which is also too wide.
It is important to note that the maximum width of coaching stock in BG is not the same as the maximum width of all railway coaches. There are some coaches that are wider than 3200 mm, but these are not standard coaches and they are not used on the Indian Railways network.