What factors determine the magnitude of the electric force between two particles ?
a. charge and distance
b. mass and distance
c. charge and density
d. mass and charge

The electric force between the two particles are calculated through the equation,
 
                          F = kQ₁Q₂ / d²

where F is the force, k is a constant called Coulomb's law constant, Q₁ and Q₂ are the charges, and d is the distance. This equation is called the Coulomb's law. 

It can be seen from the equation above that the electric forces between the objects are majorly affected by the substance's charges and distance. 

The answer to this item is therefore letter A.