What do you use to prove citizenship (born in the US)? Thanks

Where does the I-134 require proof of citizenship?

There is a section that says:

If you are not a native born U.S. citizen, answer the following as appropriate:

a. If a U.S.citizen through naturalization, give certificate of naturalization number
b. If a U.S. citizen through parent(s) or marriage, give citizenship certificate number
c. If U.S. citizenship was derived by some other method, attach a statement of explanation.
d. If a lawfully admitted permanent resident of the United States, give "A" numberThere is nothing that requires a USC to prove they are a USC.

The most commonly used items to prove citizenship are a birth certificate or passport.