Which is an element

A. h2o

B. NaCl

C. Mg

D. CO

The answer is C. Mg.

Mg is a chemical symbol of magnesium. Magnesium is an alkaline earth metal from the second group of periodic systems. Since it is a metal, in an elementary state it does not form an elementary molecule.

H2O is a chemical formula for a molecule of water.

NaCl is a chemical formula for a molecule of s odium chloride also known as salt.

CO is a chemical formula for a molecule of carbon monoxide.