(c) Pressure on left end of horizontal tube,
$p_{1} = p_{0} + h_{1}\rho g$
Pressure on right end of horizontal tube,
$p_{2} = p_{0} + h_{2}\rho\ g$
As$p_{1} > p_{2}$, so acceleration should be towards right hand side. If $A$ is the area of cross-section of the tube in the horizontal portion of U-tube, then
$p_{1}\ A - p_{2}\ A = (l\ A\ \rho)a$
Or $\left( h_{1} - h_{2} \right)\rho\ g\ A = l\ A\ \rho\ a = \frac{g(h_{1} - h_{2})}{l}$