What is Windows Forms primarily used for?

Windows Forms is primarily used as a library for building Windows client applications. It is part of the .NET Framework and provides a rich set of tools and controls to create desktop applications that run on the Windows operating system. With Windows Forms, developers can design user interfaces based on typical Windows desktop paradigms, which include forms, buttons, text boxes, and more, allowing for a powerful and interactive experience for users.

The library includes event-driven programming features, facilitating the development of responsive applications that react to user actions. Additionally, developers can take advantage of drag-and-drop features in Visual Studio to create interfaces visually, streamlining the development process for traditional desktop applications.

The other options represent areas outside the scope of Windows Forms; it is not designed for web server libraries, mobile app development, or web development graphical interfaces. This specificity makes option B the clear choice regarding the primary use of Windows Forms.