Balancing of equations is based on,

Balanced chemical equation: A chemical equation in which the number of atoms of reactants and the number of atoms of products is equal is called a balanced equation.

Every chemical equation should be balanced because:

i) According to the law of conservation of mass, atoms are neither created not destroyed in chemical reactions.

ii) It means the total mass of the products formed in a chemical reaction must be equal to the mass of reactants consumed.

Hence, the correct option is (d)