The correct option is RUNNER_OS.

GitHub Actions provides this default environment variable to reveal the operating system of the runner. It resolves to Linux, Windows, or macOS which lets you write conditional logic in your workflow steps based on the platform. While you can also read the value through the runner context using runner.os, the question specifically asks for the default environment variable and that is this one.

RUNNER_ARCH is not correct because it reports the CPU architecture such as X64 or ARM rather than the operating system.

GITHUB_RUNNER_OS is not correct because it is not a defined default environment variable in GitHub Actions and the OS value is not exposed with a GITHUB prefix.

RUNNER_DEBUG is not correct because it indicates whether debug logging is enabled for the runner and it does not provide the operating system value.

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