One end of a U-tube of uniform bore (area A) containing mercury is connected to a suction pump. Because of it the level of liquid
of density $\mathrm{\rho }$ falls in one limb. When the pump is removed, the restoring force in the other limb is: 
 

The restoring force in the tube increases when a suction pump is removed.

$\mathrm{F}=\mathrm{mg\_\_\_\_\_\_\_\_}\left(1\right)$

$\mathrm{density}=\frac{\mathrm{mass}}{\mathrm{volume}}$

$\mathrm{mass}=\mathrm{volume}\times \mathrm{density}=\mathrm{\rho A}\left(2\mathrm{X}\right)$

So, in order to balance it, it needs to be 2x for both sides.
Thus, the restoring force 

$\mathrm{F}=\mathrm{\rho A}\left(2\mathrm{X}\right)\mathrm{g}$

Hence the correct answer is $\mathrm{\rho A}\left(2\mathrm{X}\right)\mathrm{g}.$