Which of the following is an element:

The correct answer is (b), diamond.

An element is a pure substance that cannot be broken down into simpler substances by chemical means. It is made up of atoms of the same type.

Sand is a mixture of minerals, including quartz, feldspar, and mica.

Marble is a metamorphic rock composed of calcite or dolomite.

Sugar is a compound made up of carbon, hydrogen, and oxygen atoms.

Therefore, the only option that is an element is diamond.