Final answer:
The subatomic particles that have a noticeable relative mass and are located inside the nucleus of an atom are protons and neutrons, known collectively as nucleons.
Step-by-step explanation:
The subatomic particle descriptions provided suggest the particle has a noticeable relative mass and is located inside the nucleus of an atom. Based on these attributes, the subatomic particles that fit these descriptions are protons and neutrons. Both protons and neutrons, referred to collectively as nucleons, are substantial in mass and reside within the nucleus of an atom. Electrons have much less mass and are found in the electron cloud surrounding the nucleus, while neutrinos are also of negligible mass and are not found within the nucleus. Therefore, the correct answers are C. Proton and D. Neutron.