The correct option is A defined group of steps that execute sequentially on the same runner.

This choice describes a collection of steps that run one after another on the same runner within a workflow. Steps in such a group share the runner environment and workspace. Different groups can run in parallel when there is no dependency and they can also declare needs to control order across groups.

An event that triggers a workflow run is the mechanism that starts a workflow and not the structure that contains steps. Events initiate runs when something happens such as a push or a pull request.

A GitHub-hosted virtual machine used to run workflow steps describes a runner. A runner provides the execution environment but it is not the group of steps itself.

A reusable workflow called by another workflow refers to a separate workflow that can be called by other workflows. It is a distinct concept and not the unit that contains steps executed on one runner.

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