The equation that shows the conservation of atoms is the balanced one.
In this case, the balanced equation is;
2 H₂ + O₂ → 2H₂O
Because it has 4 hydrogen atoms and 2 oxygen atoms on both sides of the equation.
Chemical equations are balanced to obey the law of conservation of mass that requires the mass of the reactant to be equal to the mass of the products.
Balancing chemical equations makes the number of atoms of each element in the equation equal, thus conserving the mass in the chemical reactions.