I didn’t notice it was about his citizenship. I haven’t heard anyone question that.
The constitution [God rest its soul] requires presidents to be "natural born citizen". Arnold, for example, is not such. He is naturalized and hence cannot serve as president. It is the "naturalized" vs "natural" part of Obama that is in question.
Again, nobody questions whether he is a citizen. That is a bogus argument put forth by those who a) don't know the difference and/or b) want to distract from the real question.