Which act, passed during the Red Scare, aimed to limit the activities of radicals and labor unions?

The Smith Act, passed in 1940, was aimed specifically at curbing the activities of individuals and groups considered to be radicals, including communists and other extremist organizations. It made it a criminal offense to advocate the violent overthrow of the government, mandating that persons who were members of organizations advocating such beliefs could be prosecuted. This reflected the broader atmosphere of fear and suspicion during the Red Scare, when the government sought to suppress dissent and counter the perceived threats posed by radical ideologies.

The other acts mentioned in the options have different focuses: the Sedition Act primarily addressed issues related to speech against the government, particularly during World War I; the Espionage Act was more about prohibiting interference with military operations or support of enemies during wartime; and the Immigration Act primarily dealt with regulating immigration quotas based on national origins, rather than directly targeting radical groups or labor unions. Thus, the Smith Act is the most relevant in the context of limiting the activities of radicals and labor unions during the Red Scare.