Which is an element? ... air ... carbon dioxide ... hydrogen ... water Description

The answer is Hydrogen.

Hydrogen can be found in the periodic table. There other choices are not element but compounds, which is a molecule made of more than ONE element.

Air is a mixture of gas molecules

Carbon dioxide is CO2

Water is H2O

So as you can see, they se al made of different elements

All Elements are obtained in the periodic table & water is Not an element it is a compound.