C

In a nuclear reaction, several quantities are conserved, including:
1. Atomic Number: This represents the number of protons in the nucleus and determines the element. During a nuclear reaction, the total atomic number of the reactants is equal to the total atomic number of the products.
2. Mass Number: This is the total number of protons and neutrons in the nucleus. In nuclear reactions, the mass number is also conserved; the total mass number of the reactants equals that of the products.
3. Energy: According to the principle of conservation of energy, the total energy before and after the reaction remains constant. This includes rest mass energy and other forms of energy involved in the reaction.

Therefore, in a nuclear reaction, the atomic number, mass number, and energy are conserved. Hence, the correct answer is Option C: Atomic number, mass number and energy.