Which of the conversion factors below is the "center piece" or middle step of every stoichiometry problem? a) molar volume b) molar mass c) mole ratio d) avogadro's number

C :  Mole Ratio

here is the reason whenever we start giving answer if any question of stoichiometry problem ( how much reacted produced yield % ) these are steps 
1 . write the balanced equation 
2    Convert everything given to moles 
3.   determine limiting reagent '
4.    Convert mole limiting reagent to mole 
 other species
5.      convert moles to mass 
6.     %yield =  (  actual mass / theoretical mass ) * 100% 

Coefficient of balanced equation are mole ratio