c
\(F = F _{A B}=\frac{1}{4 \pi \varepsilon_0} \cdot\left(\frac{- Q ^2}{ r ^2}\right)\)

\(F ^{\prime}= F _{ AO }+ F _{ OB }=\frac{1}{4 \pi \varepsilon_0} \cdot\left(\frac{4 Q ^2}{ r ^2}\right)+\frac{1}{4 \pi \varepsilon} \cdot\left(\frac{-4 Q ^2}{ r ^2}\right)=4 F +4 F =8 F \quad\) [-ve sign indicates attractive force]