Answer: c) 1/4F
Explanation: In order to explain this result we must use the Coulomb force expression, that is:
F=(k*q1*q2)/d^2 where d is the distance between spheres, k a constant equal to 9*10^9N^2.m^2/C
after time each sphere lost haft of original charge and taking into account that F is directly proportional to charge of each sphere,
we have Fafter=(k*q1/2*q2/2)/d^2
then Fafter=(1/4)(k*q1*q2)/d^2=(1/4)Finitial