There are three different types of users who can mark a reply as the best answer:
- The person who asked the question
- Everybody who visits this question via the Control environment
- Users who have one of these primary roles: Super user, Moderator, Community Manager, Administrator
Who can unmark an answer?
It happens that users mark the wrong reply as an answer. Everybody who can mark something as answered can also "unmark" it again. After that, another reply can be marked as the best answer.
Tip: Want to know when you should mark something as answered? Check this topic.