Answer:
a US citizen for at least nine years.
Explanation:
In Article 1, Section 3, clause 3, of the United States Constitution, it is stated that "No Person shall be a Senator who shall not have attained to the Age of thirty Years, and been NINE YEARS A CITIZEN OF THE UNITED STATES, and who shall not, when elected, be an Inhabitant of that State for which he shall be chosen."
Hence, in this case, the correct answer is "a US citizen for at least nine years."
