What idea was the major justification for U. S. foreign policy during the Cold War era?

containment doctrine

Explanation: During the Cold War, Communism was seen as a threat which was to be thwarted. Thus intervention abroad was justified, Harry Truman invented the "containment doctrine" which was based on the claim that "communism" had to be contained.

the us constitution is a living document, meaning that it can be changed over time as the country changes.

c is the correct answer for this question.