@s.alam I have just reviewed the test (English mock test 2009 A) your child completed recently and the marking appears to be working properly. The correct answer is highlighted in green and the black tick indicates which answer you chose. If two answers were required for one question, then both answers must have been correctly selected for that answer to be marked as correct. If only one of the two answers was chosen, the answer will be marked as incorrect.
However, thanks for getting in touch about this and we will see if we can get the black tick changed to a circle or rectangle to make it easier to distinguish from an answer that is actually correct.