Final answer:
Carbon-12 (¹²C) is the species that contains only 12 nucleons in the nucleus.
Step-by-step explanation:
The species that contains only 12 nucleons in the nucleus is Carbon-12 (¹²C). Carbon-12 has 6 protons and 6 neutrons in its nucleus, with 6 electrons surrounding it. It is important to note that the number of protons in an atom determines its identity as an element, while the total number of protons and neutrons determines its atomic mass.