The correct answer is: C. large capacity transformers.
Yoke is a part of the transformer that provides a magnetic path for the flux. It is usually made of a laminated steel to reduce eddy currents. The cross-section of the yoke can be rectangular, square, or circular.
Rectangular yokes are used for large capacity transformers because they provide a more efficient magnetic path than other shapes. This is because the flux lines are more concentrated in a rectangular yoke than in a square or circular yoke.
Square yokes are used for medium capacity transformers. They are not as efficient as rectangular yokes, but they are less expensive to manufacture.
Circular yokes are used for small capacity transformers. They are the least efficient shape, but they are the easiest to manufacture.
In conclusion, yokes with rectangular cross-section are used for large capacity transformers because they provide a more efficient magnetic path.