The atomic number of an atom is the number of protons in the nucleus of an atom. It is represented by the symbol $Z$. The atomic number is the defining property of an element, as it determines the chemical properties of an element. For example, all atoms with
6 protons in their nucleus are carbon atoms, regardless of how many neutrons they have.The number of electrons in an atom is equal to the number of protons in the nucleus. This is because atoms are electrically neutral, so the positive charge of the protons must be balanced by the negative charge of the electrons.
The number of neutrons in an atom can vary. Atoms with the same number of protons but different numbers of neutrons are called isotopes. Isotopes of the same element have the same chemical properties, but different physical properties. For example, carbon-12 has 6 protons and 6 neutrons, while carbon-14 has 6 protons and 8 neutrons. Carbon-12 is the most common isotope of carbon, while carbon-14 is radioactive.
Therefore, the correct answer is $\boxed{\text{(B) Protons}}$.