Double-click on the light-blue marker of the mountain listed (among the top 100 tallest by prominence – see note below). Correct guesses will change the marker to green – incorrect will not change any marker. Complete all mountains to save your score.
Prominence is the elevation of a summit relative to the highest point to which one must descend before reascending to a higher summit. Think of it as the height of a mountain relative to the closest higher mountain. So a mountain that is the highest of anything nearby will have a high prominence, while nearby mountains that are close but not as tall will have a relatively low prominence.
If by chance you don’t see markers, try refreshing the quiz.