By definition we have that the electric force is given by:

 Where,
 k: proportionality constant
 q1: electric charge of object 1
 q2: electric charge of object 2
 d: distance between both objects
 Therefore, to increase the electric power there are two possible cases:
 1) Add more load to each object
 2) Decrease the distance between objects.
 The option that increases the electric force for this case is:
 Electrons are added to two negatively charged objects.
 This is because the values of q1 and q2 increase.
 Answer:
 A change that increases the electric force between objects is:
 B) Electrons are added to two negatively charged objects.