The correct option is GitHub Actions workflow commands because you can emit the error command from a simple run step and the runner will create an error annotation without any custom code.

These commands are parsed by the runner when they are written to standard output. They support creating error annotations and can include file paths and line numbers so you can surface precise and actionable feedback directly in the workflow run and in the Checks interface.

actions/toolkit is a library used when writing a JavaScript action. Using it would require custom code which the question specifically rules out.

Environment variables with the RUNNER_* prefix provide information about the runner environment. They do not instruct the runner to create annotations and therefore cannot generate an error annotation by themselves.

The set-env command was deprecated and later disabled for security reasons. It was used to set environment variables and not to create annotations which makes it unsuitable and less likely to appear as a correct choice on newer exams.

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