A wall of length $l$ supports water to a height H as shown in the figure. Choose the correct statement(s) (Ignore atmospheric pressure)

Consider a strip of dam at depth ‘y’
 
The force acting on the shaded strip is
 $$\begin{gathered} dF=PdA=\left(\rho gy\right)\left(ldy\right) \\ \therefore F=\underset{0}{\overset{H}{\int }}\left(\rho gy\right)\left(ldy\right)=\rho gl{\left|\frac{y^2}{2}\right|}_0^H=\frac{\rho gl{H}^2}{2} \end{gathered}$$
  Option (b) is correct.  $\Rightarrow \frac{F}{l}=\frac{1}{2}\rho g{H}^2$
 Option (a) is correct.
The torque to force dF is given by:
 $d\tau =(H-y)dF=(H-y)\left(\rho gy\right)\left(ldy\right)$
 $\therefore \tau =\underset{0}{\overset{H}{\int }}\rho gly(h-y)dy=\frac{\rho gl{H}^3}{6}$
If h be the height of point of application of force F above the base so as to produce the same torque, then $h=\frac{\tau }{F}=\frac{\rho gl{H}^3/6}{\rho gl{H}^2/2}=\frac{H}{3}$

The height of point application of resultant force below the free surface is
$H-\frac{H}{3}=\frac{2H}{3}$