Answer:
The correct answer is the last statement: the limiting of the number of people allowed to immigrate to the United States.
Step-by-step explanation:
The Red Scare that happened in 1919-1920 caused many laws to pass in the next years that limited the number of people that could enter the United States.
This anti-communist hysteria was caused by the reverberations of the October Revolution (1917) in Russia in a moment where a big part of the immigrants entering the country was from Eastern Europe. There were also many leftists (anarchists, socialists) between Spanish and Italian immigrants and many Catholics that articulated made Americans feel like these incoming people did not adhere to important American features.
The end of World War I led to generalized xenophobia and hatred for anything from another country.
The other options are wrong because they refer to events that happened before the First Red Scare (in the 1950s the US has another Red Scare thus the post-WWI Red Scare is called "first") or didn't happen at all: there were no large numbers of firearms and explosives found during the Palmer Raids.