The tendency of an element to react is closely related to
a. its atomic mass.
b. attractions between its atoms.
c. the number of valence electrons in atoms of the element.
d. the ratio of protons to neutrons in atoms of the element

The tendency  of an element  to react  is closely  related to  the number  of valence   electrons in atoms of the element (answer  C).

   Explanation

Valence  electrons  are outer  shell  electrons  of an atom.They govern the bonding   behavior  of  an element.

 when element react  to form compound, valence  electron  are involved.

Valence electrons  are involved in a reaction either by sharing  among   atoms or may be  transferred  from one  atom  to another.

For example in  formation  of NaCl bond there  is transfer of  valence electron between  Na  and Cl. Na  donate 1 valence electron  while Cl accept 1 electron to form NaCl