Hey there! I'm happy to help!
Let's look at our answer options about what events took place in the Soviet Union and China during the last months of 1949.
1. Russia's successful test of an atomic weapon.
2. The collapse of Mao Zedong's government to communist forces.
3. The death of Joseph Stalin.
4. Both the collapse of Mao Zedong's government to communist forces, and the death of Joseph Stalin.
5. All of these answers are correct.
Answer #1 is correct. The Soviet Union successfully tested their first nuclear bomb in 1949.
Answer #2 is incorrect. Mao Zedong was the founder of the People's Republic of China, and he was the leader of their communist party, and he set it all up in 1949. He was the communist and his type of government has not been removed to this day.
Answer #3 is incorrect. Stalin died in 1953, not 1949.
Answers #4 and #5 are incorrect because #2 and #3 are incorrect.
Therefore, the answer is Russia's successful test of an atomic weapon.
Have a wonderful day! :D