For which pair of objects would adding the same amount of electrons to each object result in a decrease in the electric force?
two positively charged objects
two negatively charged objects
two objects that have no charge
two objects that have opposite charges

Let us consider two charges  q and q' separated by a distance r in a medium.

Hence the force of attraction or repulsion according to Coulomb's law is -

Here is the permitivity of medium.

                        Here we see that   and

Here the medium is same and and separation distance is same.

option-1 q and q' are positive in nature

Here we are adding same number of electrons to  q and q'.Hence the magnitude of charge in both the cases will be decreased. So there is a decrease in the magnitude of force.

option -2 q and q' are negative in nature.

Here we are adding same number of electrons to q and q'. Hence the magnitude of charge is increased.So magnitude of force is increased.

option-3 q and q' are neutral.

Here both q and q' gains charge. Hence they will exert a force on each other.

Option -4 q is positive and q' is negative.

Here q  loses charge while q'gains same amount of charge.Hence the magnitude force will remain same.

Hence option 1 is right.