The correct answer is: C. Atomic number.
The atomic number is the number of protons in the nucleus of an atom.
It is unique for each element and determines the chemical properties of the element. The elements in the modern Periodic Table are arranged in order of increasing atomic number.The mass number is the total number of protons and neutrons in the nucleus of an atom. It is not unique for each element, as some elements have isotopes, which are atoms with the same number of protons but different numbers of neutrons.
The neutron number is the number of neutrons in the nucleus of an atom. It is not unique for each element, as some elements have isotopes, which are atoms with the same number of protons but different numbers of neutrons.
The atomic weight is the average mass of an atom of an element. It is not a whole number, as it is affected by the different isotopes of the element.