What does a "manifest" file in web applications primarily define?

The correct answer focuses on the role of a manifest file in web applications, specifically concerning offline storage needs. A manifest file is used to define how an application can function when the user is offline. It includes a list of resources that the browser should cache so that the application can continue to work without an active internet connection. This capability is particularly important for providing a smoother user experience in scenarios where connectivity may be intermittent or unavailable.

Manifest files allow web applications to specify which files to cache, ensuring that essential assets like HTML, CSS, JavaScript files, and other resources are accessible even when the user goes offline. By managing these resources, developers can create more resilient applications that remain usable under various network conditions.

The other options, while related to different aspects of web development, do not accurately reflect the primary purpose of a manifest file. Application configuration generally pertains to the settings and parameters that define how an application runs, while web design guidelines relate more to aesthetics and layout principles. Database structure focuses on the design and organization of data within a database, which is a separate area from what a manifest file addresses.