By definition, the electric force is given by:

Where,
q1: electric charge of object number 1.
q2: electric charge of object number 2.
d: distance between both objects
k: proportionality constant
Therefore, the magnitude of the electric force is affected by:
1) The product of the charges of the objects
2) The distance between objects
Answer:
The factors that affect strength of the electric force between two objects are:
1) The product of the charges of the objects
2) The distance between objects