Freeform Quiz - How to unselect a choosen answer?

An object with a Visited state automatically changes to Visited as soon as it is clicked. And it remains Visited unless a trigger changes it. 

Delete the Visited state. Instead, give the responses a Selected state. 

For multiple-choice questions (that is, only one option should be clicked), the buttons should be in a button set. That will automatically ensure that selecting one button deselects the other buttons in the set.

For multiple-response questions, the buttons should not be in a button set. The first time a user clicks one, it will automatically change to Selected. If they click it again, it will change back to Normal. In other words, it will toggle between those states, without the need for triggers. 

Here's more info: 

• PRIMER: Take advantage of built-in states | Articulate - Community 
• Storyline 360: Working with Button Sets | Articulate - Community