An element's atomic number is the

a. mass of the atom.
b. weight of the atom.
c. sum of the protons and neutrons in an atom.
d. number of protons in an atom's nucleus.

D. Number of protons in an atom's nucleus

Any element's atomic number represents the number of protons it contains (D). The mass, weight, and sum of protons and neutrons can change for various isotopes a single element, but any isotope of a single element has the same number of protons.

I hope this helps.