My experience is that in most cases, when asked about your nationality in a form it’s easier and normal to only give one nationality.

Like at the airport, even if I have multiple nationalities I only give one passport, I just have to make sure it’s the right passport (the one with the visa for example).

However in there case of back accounts, insurances … They ask specifically “are you a U.S. citizen ?”. In this case answering no to this question would be a clear lie.