In the updated file I provided, events 16 to 21 in function checkQuestion check if an answer is correct and if it is selected:
If it is correct & selected (event 17) or incorrect & not selected (event 21), spr_tickCross for that answer displays a tick.
If it is correct & not selected (event 18) or incorrect & selected (event 20), spr_tickCross for that answer displays a cross and the local number n is increased by 1.
At the end of the function if n = 0 (i.e. no correct answers missed or incorrect answers selected) then the score is increased by 1.
If you only want spr_tickCross to be shown for selected answers then set the spr_tickCross in events 18 & 21 to invisible.
If want spr_tickCross to be shown for selected answers and missed correct answers then set the spr_tickCross in event 21 to invisible.
Whatever you choose, it would be best to communicate to the user all the correct answers and incorrectly marked answers in some manner - perhaps reveal an icon next to the correct answers or change the text colour?