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