15. An increase in atomic number is related to an increase in atomic mass because a. more electrons are present in the atomic nucleus
b. more electrons are orbiting the atomic nucleus
c. more protons are present in the atomic nucleus
d. more protons are orbiting the atomic nucleus

Explanation:

First, let's define atomic number.

    ⇒ The number of protons in an atom.

    ⇒ This number determines the identity and properties of the atom

So, atomic number relates to protons. There is an increase in atomic number, so there must be more protons in the atom. We can eliminate choices A and B, because they pertain to electrons.

We are left with choices C and D. The difference between them is the location of the protons.

Protons are part of the nucleus of the atom: the dense, central region of the atom comprised of nucleons: protons and neutrons.

Electrons, not protons, orbit the nucleus, so  D is not correct.

We are left with choice C: more protons are present in the atomic nucleus