The correct option is It is a reusable single task that you combine in a GitHub Actions workflow and you can build your own or use community provided ones.

In GitHub Actions an action is a discrete unit of work that runs as a step inside a job in a workflow. You can create your own or use community actions from the marketplace. Actions are reusable building blocks and they can be implemented as JavaScript actions, Docker container actions, or composite actions, which makes them easy to share and to compose in many workflows.

A workflow is the YAML file that orchestrates jobs and steps and those steps invoke actions. This separation explains why the correct option describes an action as a single task that you combine to build a larger automation.

It is a prebuilt workflow that GitHub publishes for common development tasks is incorrect. GitHub does provide starter workflows as templates, yet those are full workflows and not individual actions, and actions are the atomic tasks used within steps.

It is a Google Cloud Build trigger that starts when you push to a branch is incorrect because that describes Google Cloud Build rather than GitHub Actions.

It is an integrated plugin for Visual Studio Code that runs tests during editing is incorrect because a code editor plugin is not a GitHub Actions action and actions run in workflows on runners.

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