Final answer:
Professor Plum mentions the Red Scare, which was a period of intense fear and paranoia in the United States regarding the potential threat of communism and anarchism. Option b is correct.
Step-by-step explanation:
The correct answer to the given question is b) Professor Plum mentions the Red Scare. The Red Scare refers to the period, particularly between 1919 and 1920, of heightened fear and paranoia in the United States over potential threats from anarchism and Communism.
It involved widespread concern that communists, or 'Reds,' could instigate a revolution within America as they had done in Russia in 1917. This era was marked by a series of government actions against suspected radicals and the propagation of fear over Communist infiltrators allegedly plotting to overthrow the American government.
Aspects such as the discovery of letter bombs addressed to prominent officials and the trial of Sacco and Vanzetti, who were feared as foreign anarchists, serve as illustrations of the tense atmosphere during that time.