Yard : fence : : cell : nucleus cytoplasm membrane mitochondria

The correct answer is membrane. Just like the fence protects the yard so does a membrane protect the cell. It prevents things from getting in or out unless passageways are created. The things in the yard would be the mitochondria and other organelles, while the air in the yard and the ground where all things are found would be the cytoplasm. The membrane protects what's in just like a fence protects those in the yard.