You didn't include any of answer choices but i can guarantee you its whichever one that has to do with Christopher getting distracted and not concentrating on his studying. Not taking proper notes, or just general bad studying habits.
It was just one of the MANY reasons that resulted in the 2nd WW. Maybe if they hadn't been so harsh towards Germany and blamed them for the cause of WW1 then the 2nd one most likely wouldn't have happened.
