The correct option is Use the mentions search qualifier followed by the username such as mentions:alex.

This qualifier returns issues and pull requests that include a reference to the specified user in the title, the body, or any comment. When you run it within a repository it lists every issue in that repository where the teammate is referenced.

Use the mentioned search qualifier followed by the username such as mentioned:alex is incorrect because that keyword does not exist in GitHub search and the valid form uses the plural word.

Use the author search qualifier followed by the username such as author:alex is incorrect because it filters for items created by that user rather than items that include a reference to them.

Use the commenter search qualifier followed by the username such as commenter:alex is incorrect because it finds items where the user commented which does not include issues where others referenced them without their own comment.

Open the person's profile and read their activity feed is incorrect because it is not a search qualifier and it is not a reliable or scoped way to list repository issues that reference a user.

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