The correct answer is: B. 10 percent
The yoke is the part of a transformer that connects the two legs of the core. It is made of a thicker material than the core to withstand the magnetic forces. The cross-section of the yoke is made about 10 percent greater than that of the core to ensure that it is strong enough to carry the magnetic flux.
Option A: 5 percent is too small a difference. The yoke needs to be strong enough to carry the magnetic flux, so it needs to be made of a thicker material than the core. A difference of 5 percent would not be enough to ensure that the yoke is strong enough.
Option C: 15 percent is too large a difference. The yoke needs to be strong enough to carry the magnetic flux, but it also needs to be lightweight. A difference of 15 percent would make the yoke too heavy.
Option D: 30 percent is too large a difference. The yoke needs to be strong enough to carry the magnetic flux, but it also needs to be lightweight. A difference of 30 percent would make the yoke too heavy.
Option E: none of the above is the correct answer. The cross-section of the yoke is made about 10 percent greater than that of the core.