What is a peninsula?

A peninsula is indeed defined as a large mass of land that projects into a body of water. This characteristic is key, as it distinguishes a peninsula from other landforms. It is surrounded by water on three sides, with the land connected to the mainland on the other side. This geographical feature often influences climate, wildlife, and human settlement patterns due to its proximity to water.

The other choices describe different geographical features: a small island is completely surrounded by water, underwater land refers to submerged landforms often related to continental shelves or ocean floors, and a flat plain adjacent to a river describes a floodplain or river valley, which are not characterized by being surrounded by water on three sides. The definition of a peninsula thus highlights its unique position as an extension of land into an aquatic environment.