What determines an atom's reactivity?
a) how many protons the atom contains.
b) how many energy levels the atom contains.
c) whether the atom is a metal or a nonmetal.
d) how many valence electrons the atom contains.

An atoms reactivity can be determined by how many valence electrons the atom contains.

Option D

Explanation:

The tendency of elements to react with each other depends on their electronegativity. An element having higher electronegative value will react easily with other elements. So, an electronegative elements tends to have highest reactivity.

Some of the elements in the periodic table does not react with any other elements, they are the noble elements as their electronegativity values are zero. All are elements present in a single group exhibit common electronegative.

And, the measure of electronegativity is done by the number of electrons in the valence shell or in other terms the number of valence electrons. Thus, the valence electrons count decide the reactivity of an atom.

that's what i got you can count the number of atoms

a electronegativity decreases beacuse the size of nucleus is bigger.