During Japan's time of expansion in World War One, communism was only prevalent in the Soviet Union. It wasn't spreading like it later would after World War 2. It did not want to protect it's territory from Chinese aggression. China knew that if they got in a war with Japan, they would lose in terms of area and population. Japan did not want to remain neutral either, Emperor Hirohito wanted Japan to ally with it's closest allies. So, the answer is actually option B.
