The correct answer is - a. Japan.
Hitler had his hopes that the United States would not enter the war because of the threat of Japan. Until some point during the war he was right, and the United States indeed were not willing to risk to get engaged in a war with such a dangerous opponent, but the things changed with the attack of the Japanese on Pearl Harbor. The United States were in a way forced to enter the war because of this, as it was an act of aggression over them, so they entered the war, and with the help of few other countries manged to defeat Japan. Than they were free to help the European countries fighting against Germany, and Germany found itself in a position to fight alone against everyone, so understandably lost the war in the end.