Answer:
c. They feared they would be attacked by the UN forces that had pushed too close to their border with Korea.
Step-by-step explanation:
The correct answer is c. China entered the Korean conflict and pushed the U.S.-led UN forces back into South Korea because they feared they would be attacked by the UN forces that had pushed too close to their border with Korea. China saw the UN forces' advances as a threat to their own security and sovereignty, and therefore intervened to protect their interests.