What events of 1949 thrust the cold war into a new and seemingly more dangerous stage

The first major event in 1949 was the Soviet Atomic Bomb Test. This shocked the US because it didn't expect the Soviets to have this weapon this early on. This event led to many Americans scared for their own safety. As a response, the US started expanding its weapons.

Other events include the formation of the North Atlantic Treaty Organization (NATO) by the US. The Soviets responded by forming the Warsaw Pact to counter the threats posed by NATO. 

The answer is c. good luck : )