Which of the following 'islands' is not really an 'island'?

Established in 1636 and located between Connecticut and Massachusetts, Rhode Island (officially, the State of Rhode Island and Providence Plantations) is the smallest of the 13 original colonies of the United States.

While known for its quaint seaside colonial towns and its wide sandy beaches, famous for its recreational sailing and Gilded Age mansions, such as The Breakers, it is not an island.

Historians are not entirely sure where the misnaming came from.

One theory is that the colony, originally established by religious dissidents banished by the Puritans from Massachusetts, was referred to nearby Aquidneck Island by local Native Americas, but instead ended west from where they left, and still firmly on the mainland.