Which of the following types of eggs results in a firm yolk after cooking?

The type of eggs that results in a firm yolk after cooking is over well. This cooking method involves flipping the egg and cooking it until both the white and the yolk are thoroughly set. As a result, the yolk becomes firm rather than runny, which is characteristic of this cooking style.

In contrast, over easy and sunny-side up eggs typically retain runny yolks, while over medium eggs have a slightly cooked yolk but may still be somewhat gooey. These other methods either keep the yolk partially or fully uncooked, making them unsuitable for achieving the firm texture associated with over well eggs.