Choose the word which is different from the rest.

The answer is C. Tower.

A valley is a low area between hills or mountains. A sea is a large body of salt water. A mountain is a large landform that rises sharply above the surrounding land. A river is a natural stream of water that flows in a channel on the surface of the land. A tower is a tall building or structure.

A tower is the only man-made structure in the list. The other four are all natural features.