Which cooking style results in a yolk that is solid and fully cooked?

The cooking style that results in a yolk that is solid and fully cooked is indeed the one described in the correct answer. When eggs are cooked "over hard," they are typically fried on one side, and at some point during the cooking process, the yolk is either broken or allowed to cook longer until it becomes firm. This method ensures that the yolk is solid rather than runny, providing a consistent texture and flavor throughout the egg.

In contrast, the other cooking styles do not lead to a fully cooked yolk. For instance, "over easy" eggs have a runny yolk as they are flipped only briefly, allowing the yolk to remain soft and slightly uncooked. "Over medium" results in a yolk that is partially cooked but still somewhat runny. "Sunny-side up" eggs are cooked on one side only, leaving the yolk completely uncooked and runny. This distinction in cooking techniques is essential for achieving the desired doneness of the egg yolk.