The correct option is Ubuntu Linux.

GitHub Codespaces runs a development container on a GitHub hosted Linux virtual machine by default. The default image that GitHub provides is the Dev Containers universal image which is built on Ubuntu. This means a new codespace starts with an Ubuntu based container unless the repository configures a different image in a devcontainer.json file.

Windows is not the operating system for the container in GitHub Codespaces. A codespace does not run Windows as the container operating system on GitHub hosted infrastructure.

macOS is not offered as a container operating system in GitHub Codespaces. You cannot run a macOS container in this environment.

Debian is not the default even though it is related to Ubuntu. You can choose a Debian based image by configuring a devcontainer.json file but that is not the default.

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