I am not able to understand why the values of measured theoretically and experimentally would be different and even if they differ how does it affect if they are like charges or unlike.
If two charged spheres are conductors, like charges will repel each other and move to the far surfaces of the spheres. With a larger separation distance, the measured force will be less than the point charge calculation. The effect will be opposite for unlike charges. The charges on non-conducting spheres will stay where they are placed, and can in theory be placed to give any desired result. I do not like any of the given answers. Answer b should say “may” instead of “must”.