Cobalt is comprised of 27 protons, 33 neutrons, and 27 electrons
The answer is D) a neutron.
When we say an atom's mass is, like 5 atomic mass units actually we are saying that the total number of the neutrons and protons in its nucleus is 5.
The mass of a neutron and a proton is each one atomic mass unit.
Mass of a neutron is approximately 1 atomic mass unit.
The second thesis statement is perfect. It supports the claim and presents main idea.