The correct answer is D) a US citizen for at least nine years.
<em>According to Article I, Section 3 of the Constitution, each United State's senator must be a US citizen for at least nine years.
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Another qualifications in order to be a Senator in the United States according to the Constitution, are all Senators must have <em>three qualifications: live in the state that they are representing when they are elected, be a US citizen for at least nine years, and at least thirty years of age.
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The Senators are elected by the State Legislatures, the Senate has two Senators from each state, and each Senator has one vote.