Which surface of the lung faces the hilium of the lung?

The mediastinal surface of the lung is the correct choice because this is the area that faces the mediastinum, the central compartment of the thoracic cavity. The mediastinum contains vital structures including the heart, great vessels, and trachea. The hilium of the lung, which is the area where bronchial tubes, blood vessels, and nerves enter and exit the lung, is located on the mediastinal surface. This is why the mediastinal surface is oriented towards the hilium, allowing for the necessary connections between the lung and the structures in the mediastinum.

In contrast, the costal surface faces the rib cage, the diaphragm surface is oriented towards the diaphragm on the inferior aspect, and the inferior surface, as a whole, does not specifically interact with the important anatomical structures associated with the hilium. Understanding the orientation of these surfaces is key in the study of lung anatomy and function.