Which is not a borough of New York City?
New York City is comprised of five boroughs, Manhattan, Brooklyn, Queens, Staten island, and The Bronx. A borough is an administrative unit within the New York City's government. The five boroughs were formed with the merger of the New York State counties of Kings, Richmond, New York, and parts of Queens in 1898. The Bronx though formed as NYC borough in 1898 wasn't effectively a separate county until 1914 when it became the last of the sixty-two counties in New York State. (Bronx County doesn't require a definitive article as is usually applied to the borough, "The Bronx".)
Since then all the boroughs are coextensive with their respective New York State counties. Three of the boroughs have different names than their state counties. Richmond County was the borough of Richmond until 1975 when the name changed to Staten Island, Manhattan is New York County, and Brooklyn is Kings County.