Final answer:
On the reactant side of the chemical equation, there are 4 atoms.
Step-by-step explanation:
The chemical equation 2H₂O → 2H₂ + O₂ represents the reaction between water molecules to form hydrogen gas and oxygen gas.
To determine the number of atoms on the reactant side, we can count the number of atoms of each element.
On the reactant side, there are 2 molecules of H₂O and a total of 4 hydrogen atoms and 2 oxygen atoms. Therefore, the correct answer is a) 4.