Which of the following debuggers is NOT open-source?

WinDbg is the correct answer because it is a proprietary debugging tool developed by Microsoft. It is included with the Windows SDK and primarily used for debugging applications running on Windows operating systems. Unlike open-source debuggers like GNU Debugger (GDB), which allows users to view, modify, and share its source code freely, WinDbg does not provide this level of access.

The other debuggers listed, including GDB, OllyDbg, and Immunity Debugger, are either freely available for use or have open-source versions, meaning their source code is accessible to users for modification and distribution. This open nature of those tools contrasts with WinDbg's proprietary status, which is why it stands out as the one that is not open-source. By understanding the nature of open-source versus proprietary software, one can appreciate the implications it has on user collaboration, customization, and use in diverse environments.