The statement that is always true when nuclear fusion occurs is:
The combined number of protons and neutrons remains constant.
This is because in fusion reactions, lighter nuclei combine to form a heavier nucleus, and while mass can be converted to energy (as per Einstein's equation \(E=mc^2\)), the total number of nucleons (protons and neutrons) from the reactants is equal to the total number of nucleons in the products, taking into account any energy released.