d. isotopes

Explanation:Most elements as they occur naturally on earth are mixtures of several isotopes.

For example:* The element hydrogen has atoms that have one proton in the nucleus. This is the most common form of hydrogen. They have an atomic number of one due to the presence of single proton in the nucleus, and a mass of one due to the presence of a single proton.

* Occasionally, one finds an atom of hydrogen in which a neutron has been incorporated into the nucleus. This isotope of hydrogen is characterized by an atomic number of one (only one proton; hydrogen can only have one proton in the nucleus), but an atomic mass of two (one proton plus one neutron). This isotope of hydrogen is known as "deuterium" or heavy hydrogen.

* Finally, even rarer is the third isotope of hydrogen [3H]. It has a mass of three which can only occur from the presence of two neutrons in the nucleus. This final isotope of hydrogen is known as "tritium" and it is radioactive.