Solution
When charged spheres interact, the charges do not remain uniformly distributed due to electrostatic induction (or polarization in insulators).
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Like Charges (Repulsion): The charges repel each other and redistribute to the farthest surfaces of the spheres. This increases the effective distance between the charge centers ($r_{eff} > r_{center}$). Since $F \propto 1/r^2$, the measured repulsive force ($F_m$) will be less than the calculated force ($F_c$) which assumes point charges at the center.
Result: $F_c > F_m$. -
Unlike Charges (Attraction): The charges attract each other and accumulate on the inner surfaces (closest to each other). This decreases the effective distance ($r_{eff} < r_{center}$). Therefore, the measured attractive force ($F_m$) is greater than the calculated force ($F_c$).
Result: $F_c < F_m$.
This phenomenon occurs for both conducting and insulating spheres (via polarization), making the conclusion certain irrespective of material.
Answer: (d)