What is used on areas in direct contact with the ground to keep moisture out of the structure?

The correct choice is the vapor barrier. A vapor barrier is designed to prevent moisture from passing through walls, floors, and ceilings, particularly in areas where moisture from the ground can infiltrate a structure. This is crucial in maintaining a dry indoor environment, as excess moisture can lead to issues such as mold growth and structural damage.

In contrast, a moisture barrier typically refers to materials that mitigate the movement of liquid water rather than vapor. Insulation is primarily used to reduce heat transfer, and while it may have some moisture-resistant properties, it is not specifically designed to manage moisture intrusion. Waterproofing sealants are used to protect surfaces from liquid water, but they are not strictly meant for areas in direct contact with the ground to control moisture vapor.