I dislike this question with a vengeance because it contains double negatives.
The critical word is "cannot":
If these statements are true, only one of
the sentences below cannot be true.
Maddy got half Ben's mark, and Ben scored the same as Arif.
We are told that answers are either right or wrong, which implies that there are no half marks.
Thus, Maddy score was any of 1, 2, 3, 4 or 5.
Ben and Arif therefore scored any of 2, 4, 6, 8, 10.
Arif therefore could not have got 9 and the answer is B.