What is primarily stored in the res/layout folder of an Android project?

The primary purpose of the res/layout folder in an Android project is to store XML files that define the user interface layouts. These XML files describe the arrangement of UI components, such as buttons, text views, images, and other widgets that make up an application's screen layout. By using XML, developers can separate the design aspects of an application from the business logic, making it easier to manage and update the UI.

This effective organization allows developers to create multiple layouts for different device configurations, ensuring that the user experience is optimized across various screen sizes and orientations. For instance, developers might include a different layout for tablets compared to phones.

While other resources like drawable graphics or strings do exist in their respective folders, the specific function of the res/layout folder is solely focused on layouts defined in XML format, which directly impacts how the app's user interface is displayed. Therefore, the inclusion of XML files defining layouts makes this option the correct answer.