Which of the following operating systems is available for free and can be modified by anyone?

Windows is a proprietary operating system developed by Microsoft, which is not free and cannot be freely modified by users.

Linux is an open-source operating system kernel, and its various distributions (like Ubuntu, Fedora, Debian) are widely available for free. The open-source nature allows anyone to view, modify, and distribute its source code, fostering extensive community development and customization.

Mac OS is a proprietary operating system developed by Apple for its hardware, which is not free and cannot be freely modified by users.

While Chrome OS is Linux-based and uses open-source components, the full Chrome OS distribution as provided by Google is not entirely open for modification by anyone, and it typically comes pre-installed on specific hardware.