Triggers on question slides

When an item has a Normal state and a Selected state, it will automatically toggle between the two when clicked. Thus, you don't need any triggers to change the state of the buttons. (In fact, adding triggers that try to perform the built-in functionality can cause issues.)

It'd also be much easier to use the built-in Question functionality. To get the custom look you want, start with the items set up on a regular slide. Then convert it to a Freeform - Pick Many question. Storyline will turn it into a Graded Question slide, with a Form View where you can indicate which items are selectable and which are correct. The program will score it like a regular Graded Question slide. 

• Here's more info about Freeforms: https://community.articulate.com/series/articulate-storyline-360/articles/articulate-storyline-360-user-guide-how-to-convert-existing-slide-to-freeform-interaction