"One" is the subject. (singular) I'd be inclined to call "about" an adjective in this sentence.
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Anonymouswith 1 in 3 being the prepositional phrase,Anon, I realize you didn't ask about "about," but for some reason when I read "1 in 3 being the prepositional phrase" I thought you meant "about 1 in 3."
AvangiSo based on the consensus reasoning, it would be correct to say, In the US, about one person in five are an immigrant. Well, maybe "about one person in five are immigrants."Yes, the latter.