Submit button Override

TuwannaSycamore​: To override the default "Invalid Answer" message, you have to prevent the program from submitting the question for scoring if the user hasn't clicked any of the responses.

Thus, the trigger would be something like this: 

Submit [interaction] when the user clicks Submit

If the state of Check box 1 = Selected
or the state of Check box 2 = Selected
or the state of Check box 3 = Selected
or the state of Check box 4 = Selected

Else show layer Invalid Response

With that trigger, if any response is Selected, the program will perform the Submit [interaction], and it will show the appropriate feedback layer. 

If none of the responses is Selected (that is, none of the conditions are met), the program won't perform the Submit [interaction] action. Instead, it will show your custom Invalid Response layer.