Which idea is mostly associated with the red scare of the 1920s

The correct answer is:

Radicalism

Radicalism is the belief that society needs to be changed, and that these changes are only possible through revolutionary means. As the Cold War between the Soviet Union and the United States intensified in the late 1940s and early 1950s, hysteria over the perceived threat posed by Communists in the U. S. became known as the Red Scare. (Communists were often referred to as "Reds" for their allegiance to the red Soviet flag.)