An atom's electrons have a great effect on
a. the mass of the atom.
b. what element the atom belongs to.
c. how chemically reactive the atom is.
d. electrons really don't affect atoms that much.

The correct answer is C. How chemically reactive the atom is.

Explanation:

An atom contains a nucleus with protons and neutrons as well as electrons negatively charged and bounded to the nucleus. Electrons do not affect the mass of the atom defined by the neutrons and protons, also they do not have and influence on the element the atom belongs to that is defined by protons but they affect magnetism and reactivity.

In terms of reactivity, this is mainly influenced by the number of electrons in the external shell of the atoms as this can form bonds with other atoms and therefore had a chemical reaction. Thus, an atom's electrons have a great effect on how chemically reactive the atom is.