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

I am pretty sure that the answer is A, because the element's atomic number is how many protons, maybe the neutrons, it has in it and all elements need 18 protons to make it stable. So the answer is A.

Hope that helps.

The correct answer would be 1. precipitation