The correct answer is A. The number of protons in its nucleus.
The chemical behavior of an atom is determined by the number of protons in its nucleus. This is because the number of protons determines the element that the atom is.
For example, all atoms with 1 proton in their nucleus are hydrogen atoms, all atoms with 2 protons in their nucleus are helium atoms, and so on.The number of neutrons in the nucleus does not affect the chemical behavior of the atom, as long as the number of protons remains the same. This is because neutrons do not participate in chemical reactions.
The number of electrons orbiting around the nucleus does affect the chemical behavior of the atom, but only to a small extent. This is because the electrons are arranged in shells around the nucleus, and only the electrons in the outermost shell are involved in chemical reactions.
The number of nucleons in the nucleus is the sum of the number of protons and neutrons in the nucleus. It does not affect the chemical behavior of the atom.