The fusion of two isotopes of hydrogen typically refers to the fusion of deuterium (²H) and tritium (³H). However, in simpler terms, if two deuterium nuclei were to fuse, they could indeed produce a neutron and helium-4 (⁴He) as part of the reaction.
Based on the options you've provided, the closest correct answer to the fusion reaction resulting in a neutron and a larger element would be:
D. helium.
In summary, when two deuterium nuclei undergo fusion, they can produce a neutron and helium, which would align with option D.