The correct answer is B) the early successes of North Korea.
What prompted China to enter the war directly was the early successes of North Korea.
It seemed that China could be a natural ally to North Korea after the result and composition of the international order after World War II.
Under the rule of Mao Ze Dong, China had changed its government to Communist ideology. The Soviet Union was a primary ally of China and heavily influence the entrance of Communism under the rule of Mao.
So on November 25, 1950, Mao decided to enter the Korean War supporting North Korea. The Chinese troops aggressively attacked troops from the United Nations and the United States. General Douglass never saw it coming but to his surprise, more than 250,000 soldiers had been sent to attack the troops at his command.