Hey there!
The atomic number is the number of protons in the atom, so A is incorrect.
In a neutral atom, which we assume unless otherwise stated, the number of protons is equal to the number of electrons, so B is incorrect.
The number of protons is what determines the name and identity of an element, so C is incorrect.
The only thing it doesn't tell us is the number of neutrons, because one kind of atom can have different isotopes.
So your answer is D) The number of neutrons in an element.
Hope this helps!