A rectangular block has a square base measuring $\mathrm{a}\times \mathrm{a}$, and its height is h, It moves with a speed v on a smooth horizontal surface

Given, 

Rectangular block, square base = $a\times a$

height = h

speed = v

We know A body topples due to an unbalanced torque about its centre of mass.
But here, there are is two forces acting on block and they are Mg and N.

 Mg & N both passes through the centre of mass and hence produce no torque.
So, block will not topple for any value of V.

Hence the correct answer is it will not topple for any value of v.