Answer: D. Atomic Number
Explanation:
Atomic number of an element is the number of protons in the nucleus of an atom the element. An atom has three fundamental particles: protons, electrons and neutrons. Protons and neutrons are found in the nucleus of an atom while electrons are found outside the nucleus. The atomic number of an element determines the properties of the element. No two atoms of different elements have the same atomic number, but two or more atoms of the same element can have the same atomic number but different atomic masses. When atoms of the same element have the same atomic number but different atomic masses, they are said to be isotopes.
