Answer:
The correct answer is the last option: the limiting of the number of people allowed to immigrate to the United States.
Explanation:
The Red Scare that took place in America after World War I is sometimes called First Red Scare because of the most famous anti-communist surge that happened in the 1950s.
This First Red Scare was an articulation of xenophobia generated by the end of World War I (1914-1918) and anti-communism intensified by the repercussion of the October Revolution (1917) in Russia.
The peak of immigration to the US happened in the turning of the 19th to the 20th century but it generated mass fear only after World War I because of the factors cited above.
Eastern immigrants, Spanish and Italian, who were politicized and fought for their rights as workers besides a few of them being Catholics, were seen as threats to America's most important values. Because of this, the result of the Red Scare were many laws that limited immigration.
All other options refer to past events, except the discovery of firearms and explosives in the Palmer Raids which never happened.