The incident that fueled the Red Scare across the United States in the 1920s was:
A. A bomb sent by an anarchist group exploded in the Oklahoma state senate, killing four legislators.
During this time period, there was a rising fear and paranoia about the spread of communism and anarchism in the United States. The bombing incident in the Oklahoma state senate, carried out by anarchists, intensified these fears and led to a widespread crackdown on perceived radical and leftist elements in society. This event contributed to the atmosphere of suspicion and hostility known as the Red Scare, where many Americans were targeted and persecuted for their political beliefs.