The correct answer is: A. upto 20 A.
Cylindrical windings using circular conductors are used for current ratings of up to 20 A. This is because the circular conductors have a lower resistance than other types of conductors, such as square or rectangular conductors. This means that they can carry more current without overheating.
The other options are incorrect because they are higher current ratings than the cylindrical windings can handle. If you try to use a cylindrical winding with a current rating that is higher than its capacity, it will overheat and could cause a fire.
It is important to choose the right type of winding for your application. If you need to carry a high current, you should use a winding with a higher current rating. However, if you only need to carry a low current, you can use a winding with a lower current rating. This will save you money and resources.