Which event pushed the Western Allies to seek a military alliance?

a. the successful detonation of the Soviet Union’s first atomic bomb
b. the refusal of Soviet Union to accept financial aid under the Marshall Plan
c. the communist takeover of the Czech government with Soviet support
d. the complete demilitarization of East Germany by the Soviet Union

You might be tempted to choose the first answer, about the Soviet detonation of its first atomic bomb. However, the Western Allies formed the military alliance known as NATO in April 1949, and the Soviets did not detonate their first atomic bomb till August of 1949.

The communist coup in Czechoslovakia was a main cause of concern that led to the formation of the North Atlantic Treaty Organization (NATO).

The Office of the Historian of the United States lists these events of 1947-48 as strong influences that led to the formation of NATO:

Ongoing civil war in Greece and tensions in Turkey (which led to the Truman Doctrine to provide economic and military aid to any nation struggling against communist aggression)
The Soviet-sponsored coup in Czechoslovakia
Elections in Italy, where the communist party had made significant gains
The Soviet blockade against West Berlin

The Office of the Historian reports: "These events caused U.S. officials to grow increasingly wary of the possibility that the countries of Western Europe might deal with their security concerns by negotiating with the Soviets. To counter this possible turn of events, the Truman Administration considered the possibility of forming a European-American alliance that would commit the United States to bolstering the security of Western Europe."