The tendency of rotation will be about the point \(C\).
For minimum force, the torque of \(F\) about \(C\) has to be equal to the torque of \(mg\) about \(C\).
\(\therefore F \left( a \frac{\sqrt{3}}{2}\right)= mg \left(\frac{ a }{2}\right) \Rightarrow F =\frac{ mg }{\sqrt{3}}\)