According to Article I, Section 3 of the Constitution, each senator must be an adult who is at least 25 years old. a lifetime resident of Washington, DC. a registered voter in one’s state. a US citizen for at least nine years.

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."

articles of confederation