Deuterium is an isotope of hydrogen. the nucleus of a deuterium atom consists of one proton and one neutron. when two deuterium nuclei fuse, helium-3 is formed, and a neutron is emitted. which equation illustrates this process?

The last option shows the fusion of two deuterium nuclei and the consequent formation of helium-3 and the emission of a neutron. The superscripts at the left side of the elements denote their atomic mass which is approximately equal to the number of protons and neutrons. On the other hand, the subscripts refer to the number of protons. In the correct equation, deuterium has 1 proton and 1 neutron while helium has 2 protons and 1 neutron which is shown in the last option.  

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