The correct option is A commit is a discrete set of file changes saved on a branch, and any new commits pushed to that branch are automatically included in the open pull request.

This is correct because a commit records a specific snapshot of changes in the repository history and it is associated with a branch that references it. On GitHub a pull request tracks the head branch and compares it to a base branch. When you push additional commits to the same head branch during review the open pull request updates automatically to include those new commits.

A commit is a request to merge one branch into another is incorrect because a request to merge is a pull request while a commit records changes and does not propose or perform a merge by itself.

A commit can only be created after the pull request has been merged is incorrect because you create commits independently of pull requests and you typically create them before opening one. You can also create commits after a pull request merges on any branch.

A commit is only a notification sent to collaborators about proposed changes is incorrect because a commit stores content and metadata in the repository. Notifications may be sent by the hosting platform but they are not what a commit is.

A commit is a snapshot that belongs to the pull request instead of the branch is incorrect because commits belong to the repository history and are referenced by branches. A pull request simply points to a branch and reflects whatever commits are on that branch.

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