Monty, small mobile devices do not have a "Full" Browser with all the format generating features (essentially browser lite's). You need to log in on a machine with a full browser client like a PC, Laptop or Mac using Edge, Firefox, Chrome or Safari that will properly generate the "Best Answer' button.

you have to be logged on using a computer (phoned and tablets don't have the best answer button ) on each answer there should be a best button (blue) just select that button you can chose more than on answer by opening the question in multiple windows/tabs before selecting a best answer

It's a green button it should work now try it

Should be a little button to click stating "best answer."