Where must water-resistant gypsum backing board not be used?

Water-resistant gypsum backing board is specifically designed for use in areas where moisture is likely, such as bathrooms and kitchens, but it has limitations regarding direct water exposure. The correct option indicates that the material should not be used in specific scenarios.

In shower compartments, while a vapor retarder is present, water-resistant gypsum backing board is not suitable because it can still be compromised by direct contact with water over time, leading to potential mold and structural failures.

Regarding areas of high humidity, water-resistant gypsum backing board may seem appropriate, but it is not ideal for locations where it could be continuously damp or exposed directly to water, as this could eventually lead to degradation.

When considering the scenarios in the given options, it becomes clear that water-resistant gypsum backing board has clear restrictions on use where there is direct exposure to water, supporting the idea that it should not be used in those conditions. Thus, the best conclusion is that water-resistant gypsum backing board should not be used under the conditions mentioned in all scenarios provided.