Find the minimum value of $F$ for the block to topple about an edge.

When the block is about to topple the normal reaction $N$ shifts to the edge through $O$. FBD during toppling taking torque about $O$

$F\left(b\right)=Mg\left(\frac{a}{2}\right)\Rightarrow F_{min}=\frac{Mga}{2b}$.