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

Valley
Sea
Tower
Mountain E. River

The odd one out is C, Tower.

A valley is a low area between hills or mountains. A sea is a large body of salt water that covers much of the Earth’s surface. A mountain is a large landform that rises sharply above the surrounding land. A river is a large natural stream of water that flows in a channel to the sea or another river. A tower is a tall building with a narrow base and a large top.

All of the other options are natural features, while a tower is a man-made structure.