Which of the following statements correctly describes the concept of atomic number, mass number, and atomic mass?

A) Atomic number refers to the number of protons in an atom's nucleus, mass number refers to the sum of protons and neutrons in the nucleus, and atomic mass refers to the average mass of all isotopes of an element.
B) Atomic number refers to the number of neutrons in an atom's nucleus, mass number refers to the sum of protons and electrons in the nucleus, and atomic mass refers to the total mass of all isotopes of an element.
C) Atomic number refers to the number of electrons in an atom's nucleus, mass number refers to the total number of protons and electrons in the nucleus, and atomic mass refers to the mass of a single isotope of an element.
D) Atomic number refers to the number of protons in an atom's nucleus, mass number refers to the sum of protons and neutrons in the nucleus, and atomic mass refers to the total mass of an atom including electrons.