A

Joseph McCarthy gained public attention by claiming that communists and spies sent by the USSR had infiltrated the US government.

Explanation:

During the Cold War, there was a rise in propaganda against communism around the USA. The House Un-American Activities Committee, known as the HUAC, existed since 1938. Their principal goal was finding and removing citizens with ties to communism from their government and civil positions. Senator McCarthy advanced this idea and gave his name to the making of allegations against people regardless of proof, which became known as McCarthyism.

McCarthy’s continuous attempts to find and remove communists from the army led to the McCarthy hearings. Ray Cohn, more famous as Donald Trump’s future personal lawyer, was Senator McCarthy’s counsel during this trial, while Joseph Welch opposed them as a representative of the army.
Despite the results of the McCarthy hearings, future presidents, such as Dwight D. Eisenhower and Richard Nixon, carried on the spirit of McCarthyism.

Senator McCarthy Holding a Newspaper Embodying His Drop in Popularity that Followed the McCarthy Trials.
Senator McCarthy Holding a Newspaper Embodying His Drop in Popularity that Followed the McCarthy Trials (Source: https://www.historyonthenet.com)