What Marked the Beginning of the Cold War?

The answer is A. Truman Doctrine. Here's why:

The Truman Doctrine, announced in 1947, signaled a shift in U.S. foreign policy towards containing the spread of Soviet influence. This policy of containment, aimed at preventing the spread of communism, is considered the defining characteristic of the Cold War.

While events like the bombing of Pearl Harbor and the Treaty of Paris were significant historical moments, they didn't directly spark the ideological and geopolitical rivalry that characterized the Cold War. The Truman Doctrine, with its focus on containing Soviet expansion, marked the beginning of this era of tension between the United States and the Soviet Union.