Which of the following operating systems is considered closed source?

Ubuntu is an open-source operating system based on the Linux kernel. Its source code is freely available for anyone to view, modify, and distribute, which stands in stark contrast to closed-source systems. The open-source nature of Ubuntu encourages community collaboration and transparency.

While Android is based on the open-source Linux kernel, it is considered a mixed-source operating system because many of its components, particularly those developed by Google, are closed-source. However, the core of Android allows for open-source development, making it less strictly classified as closed source compared to OSX.

CentOS is another open-source operating system derived from Red Hat Enterprise Linux (RHEL). It provides a free and open-source environment, allowing users to access and modify its source code. This open-source status is fundamentally different from the closed-source model of OSX.