The answer is letter A.
Counting the atoms in each substance in the reactants and
the products is a step to balancing a chemical equation. Chemical equations
need to be balanced so that they follow the law of conservation of mass (matter
can be changed from one form to another but the mass remains constant). Usually
balancing equations require a process of trial and error to get the number of
atoms of elements in the reactants side to equal the products side.
