The correct answer is B. 6 mm.
The width of the duct in a D.C. machine is usually 6 mm. This is because the duct needs to be wide enough to accommodate the windings, but not so wide that it takes up too much space in the machine. The windings are the coils of wire that carry the current that powers the machine. They need to be able to fit snugly in the duct so that they don’t overheat. If the duct is too wide, the windings will not be able to fit properly and they may overheat.
The other options are incorrect because they are not the standard width of the duct in a D.C. machine. Option A is too narrow, and option C and D are too wide.
In addition to the width, the duct also needs to be the correct length and height. The length of the duct needs to be long enough to accommodate all of the windings, and the height of the duct needs to be tall enough to allow the windings to be installed without touching the sides of the duct.
The duct is made of a material that is a good conductor of heat, such as copper or aluminum. This helps to keep the windings cool and prevents them from overheating.
The duct is also insulated to prevent the windings from coming into contact with each other. This helps to prevent short circuits and other electrical problems.