What is one responsibility that is only for United States citizens?

Two responsibilities are RESERVED ONLY FOR U.S.

Citizens: (1) SERVE ON A JURY — citizens may be called for jury duty; lawful permanent residents and visa holders are not eligible; (2) VOTE IN A FEDERAL ELECTION — only citizens can vote in federal elections (some local jurisdictions allow non-citizens to vote in local elections, but federal elections require citizenship). Either answer is acceptable on the USCIS test. RIGHTS that are also only for citizens: voting in federal elections, running for federal office, applying for a U.S. passport. RESPONSIBILITIES that apply to all PERSONS in the U.S. (not just citizens): paying federal taxes (on income earned in U.S.), obeying the law, registering for the Selective Service (if male, 18-25, regardless of citizenship status). RIGHTS that apply to all persons in the U.S.: freedom of expression, freedom of religion, freedom to petition the government, freedom of assembly, the right to bear arms.