Step-by-step explanation:
In a chemical reaction, when mass is conserved , the number of atoms or moles of the reactants must be equal to the number of moles or atoms in the products side.
From the diagram, we should carefully look to see if the number of atoms that makes up the reactants are equal to those on the product side.
For example:
A + B → AB
Here, mass is conserved because, on the reactant side, we have 1 atom of A and on the product side we have 1 atom of A
For B, on the reactant side, we have 1 atom of B and on the product side, we have 1 atom of B.