This is bizarre. The info provided in the question was that Marty ate more than Luis, the question was how would that be possible given that Mario ate 4/6 of his while Luis ate 5/6 of his. The answer the kid wrote (Marty’s pizza was bigger than Luis’) is the only possible correct answer.

The grader is asserting that the information given in the question was wrong and that “actually it was Luis who ate more pizza”–even though it stated as a premise that “Marty ate more”. How are you supposed to give a correct answer on a test if you are expected to accept one premise (proportion of pizzas eaten) while disregarding another premise (Marty ate more than Luis)? How do you decide which part to disregard? Would they have accepted the answer, “Luis actually only ate 3/6 of his pizza, not 5/6)”? Wouldn’t that be just as valid an answer as “Marty actually didn’t eat more than Luis”?