All atoms of the same element have the same number of protons in their nucleus. This is because the number of protons, also known as the atomic number, defines the element itself. For example, all carbon atoms have 6 protons in their nucleus.
While the number of protons is fixed for a given element, the number of neutrons can vary, leading to different isotopes of the same element. Similarly, the number of electrons can change, resulting in ions, but the number of protons remains constant.
Therefore, the correct answer is c) protons.