면 틀리다

if you do that, it's wrong

A conditional "-(으)면" plus 틀리다, "be wrong, be incorrect". It marks the line between right and wrong answers: answering that late counts as wrong, saying the sentence this way is incorrect, if you do it individually the ones who get it wrong don't get to take it. It also covers something looking different from another angle. Careful with 틀리다 versus 다르다 — 틀리다 is "incorrect", though Korean speakers commonly use it where 다르다 "different" is the standard choice, which is what one line here does.