Its legal to ask if are you citizen of us?

Answer 1

Asking if you're a citizen or not is technically legal.

It does open a big can of worms, thought, because it exposes the person asking to a possible discrimination lawsuit.

For example, a potential employer will generally ask if an applicant is legally able to work in the US instead. This is a perfectly legitimate question (in fact, they're actually required to ask that employees prove their right to work in order to avoid breaking the law themselves).