Find the odd number / letters / word from the given alternative:

The odd one out is island. A bay is a large inlet of the sea between headlands or promontories, a cape is a point of land that projects into a body of water, and a peninsula is a piece of land that is surrounded by water on three sides. An island is a piece of land that is surrounded by water on all sides.

Bays, capes, and peninsulas are all landforms that are connected to the mainland. Islands, on the other hand, are not connected to the mainland. This is why island is the odd one out.