Are you referring to the I-9 form?
Until about a few years ago, the I-9 selection was for "US citizen or US national", and the courts have held that it isn't a claim to US citizenship. It started having a separate choice for "US citizen" and another for "US national" about 3 or 4 years ago.
Did your parents get US citizenship before you turned 18? If yes, that may be an option for claiming your citizenship.