The correct option is Any variable prefixed with GITHUB_ or RUNNER_.

Variables with these prefixes are reserved by GitHub and the runner. They are system defined values and are protected so attempts to change them using the GITHUB_ENV file will not take effect. This ensures the workflow execution environment remains consistent and secure.

The option CI is not covered by the reserved prefix rule. Although it is set by default, it can be changed within a workflow or by writing to the environment file for later steps in the same job, so it is not the option that cannot be changed.

The option NODE_OPTIONS does not use a reserved prefix and can be set when needed, so it is not the option that is blocked from being changed with the environment file.

The statement All default environment variables can be overridden is incorrect because some defaults are reserved and GitHub prevents overriding those, particularly the ones that use the reserved prefixes.

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