The correct option is A README.md in a repository that matches the username.

GitHub supports a special profile repository that must be named exactly the same as your account. You create it as public and place a README.md at the root. GitHub then displays that profile README at the top of your user profile and it updates automatically when you push changes. This is the intended way to add an introduction.

Only account settings can change the profile is incorrect because settings let you edit your name, bio, location, and other metadata but they do not create a custom introduction section. The introduction is powered by the profile README.

A CONTRIBUTING.md in the profile repository is incorrect because a CONTRIBUTING file provides contribution guidelines for a project. It is shown within repositories for collaborators and it does not render on a user profile.

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