A chemical reaction is a product formation through the reactants. In a reaction the number of atoms, molecules, and mass are conserved. Thus, option D is correct.
A chemical reaction involves a reactant on the left side of the reaction equation that changes to produce reactants. In a reaction, the conservational law of mass is always followed.
The number of atoms and the molecules involved in a reaction is also the same in the reactants and the products formed as can be seen from the stoichiometry coefficient of the balanced equation.
Therefore, option D. mass, atoms, and molecules are conserved in a reaction.
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