What legislative act made it more challenging for immigrants to gain U.S. citizenship?

The Naturalization Act is the correct choice because it specifically established the legal framework and requirements for becoming a U.S. citizen, and its provisions made the process more difficult for immigrants. Initially, the Naturalization Act of 1790 allowed free white persons of "good character" to apply for citizenship after two years of residency. However, subsequent amendments and acts, particularly during periods of heightened national security concerns, raised the residency requirement and added language that made it more challenging for non-white immigrants and those from certain countries to obtain citizenship.

This act directly influenced the immigration landscape by imposing stricter eligibility criteria, thus illustrating a pattern of legislative action aimed at controlling the demographic composition of the U.S. population regarding citizenship.