Do non-theatrical movies count?
Anyway, one of the movies I found had a limited theatrical release, while the other had a wide release.
The Californians (2005): C. Might be the most belly I have seen from Kate Mara apart from her other film “Shooter”. Since she was playing an environmental advocate, Kate Mara wore these short tops that either showed off her lower midriff or they rode up to expose her belly button.
Stay (2005): A or C. This one is more difficult to categorize. There is a brief scene where Elizabeth Reaser shirt rides up to expose her belly button. However, the lighting actually shows how deep and detailed her belly button is for that limited time.