Answer:
The correct answer is D. nucleons.
The law of conservation of nucleon number, also known as the law of conservation of mass number, states that the total number of nucleons (protons and neutrons) in a closed system remains constant before and after a nuclear reaction. This means that the sum of the mass numbers of the reactants must be equal to the sum of the mass numbers of the products. Therefore, the total number of nucleons is conserved in a nuclear reaction.
So, the law of conservation of nucleon number says that the total number of nucleons (protons and neutrons) is conserved before and after the reaction.