Which of the following operating systems is known for its closed-source structure?

Windows is recognized for its closed-source structure, meaning that its source code is not publicly available for users to view, modify, or distribute. This closed-source model allows the company behind Windows to maintain control over the operating system, ensuring that it adheres to specific standards and security protocols. Users must comply with licenses that do not grant them the freedom to inspect or alter the code, which can lead to less transparency regarding its functioning compared to open-source systems.

In contrast, operating systems like Linux, Debian, and Ubuntu are built on open-source principles, allowing users to access and modify the source code freely. This fosters a collaborative environment where developers across the world can contribute to the software, enhance its features, and improve security through peer review and community-driven initiatives. Thus, the nature of these operating systems is fundamentally different from that of Windows, which is why they are not classified as closed-source.