'Mark as Best Answer' should appear immediately above the relevant text. However you'll only see it when:
(a) you're logged in ; and
(b) viewing a thread which you started yourself.
If you don't see it then it must be a problem with your web browser.
So, which browser are you using (Internet Explorer, Edge, Firefox, Chrome, Dolphin, Safari, Opera, etc) and on what type of device (Windows PC, Mac, iPad, Android tablet, etc)?