Question

which of the following is an element?
o protons
o carbon dioxide
o hydrogen
o water

Explanation:

An element is a pure substance made of only one type of atom. Protons are sub - atomic particles, not elements. Carbon dioxide ($CO_2$) and water ($H_2O$) are compounds made of different types of atoms. Hydrogen is an element with the symbol H and is made of hydrogen atoms only.

Answer:

Hydrogen