c
(c) Initially, force between \(A\) and \(C\) \(F = k\frac{{{Q^2}}}{{{r^2}}}\)
When a similar sphere \(B\) having charge \(+Q\) is kept at the mid point of line joining \(A\) and \(C\), then Net force on \(B\) is \({F_{net}} = {F_A} + {F_C}\) \( = k\frac{{{Q^2}}}{{{{(r/2)}^2}}} + \frac{{k{Q^2}}}{{{{(r/2)}^2}}} = 8\frac{{k{Q^2}}}{{{r^2}}} = 8F\)
(Direction is shown in figure)
