Highlighting Options?

I have it set up to allow multiple options to be selected. The problem is that it is a schematic drawing and I have boxes around specific options on the image that they have to choose from.  I am trying to have each box that the user selects show that it was selected, highlighted in some way. That way the user doesn't reselect the box causing the issue you stated above.  I am not sure how to do that?  They would need to choose five out of the eight boxes on the image to get the question correct.  Currently, once the user selects a box there is nothing that tells them they already selected that box.  I can have the box pulse when clicked on, but it doesn't stay pulsing to indicate that it has already been chosen.  There is my dilemma, how do I make each selection stay highlighted so the user knows what which options they have already selected. 

Thank You for your assistance!