I don't think much of the question. It should be 'For his mistake, he won't get full marks' or 'For his mistake, he won't get the maximum mark/grade'. Your answer should be 'If he hadn't made a mistake, he would have got full mark s '.
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essam gaweeshIf he hadn't made a mistake, he would get the full mark. What do you think of my answer?As Rover_KE said, it's not quite right. As he said, you need "would have got" in the second clause.