To answer this question, we need to understand the composition of a helium nucleus and how adding a neutron affects it.
A helium nucleus consists of 2 protons and 2 neutrons. When one neutron is added to a helium nucleus, the total number of neutrons becomes 3, while the number of protons remains the same at 2.
Therefore, the resulting nucleus is still a helium nucleus, with 2 protons and 3 neutrons. This means that the correct answer is B) Helium.