b
Rate of flow $=\frac{\mathrm{dQ}}{\mathrm{dt}}=\mathrm{A}_{1} \mathrm{V}_{1}=\mathrm{A}_{2} \mathrm{V}_{2}$
Apply energy conservation
$\mathrm{P}_{1}+\frac{1}{2} \rho \mathrm{V}_{1}^{2}+\rho \mathrm{gh}_{1}=\mathrm{P}_{2}+\frac{1}{2} \rho \mathrm{V}_{2}^{2}+\rho \mathrm{gh}_{2} ; \mathrm{h}_{1}=\mathrm{h}_{2}$
$\mathrm{P}_{1}+\frac{1}{2} \rho \mathrm{V}_{1}^{2}=\mathrm{P}_{2}+\frac{1}{2} \rho \mathrm{V}_{2}^{2}$
$\mathrm{P}_{1}-\mathrm{P}_{2}=\frac{1}{2} \rho\left(\mathrm{V}_{2}^{2}-\mathrm{V}_{1}^{2}\right)$
$\mathrm{V}_{2}^{2}-\mathrm{V}_{1}^{2}=2 \mathrm{gh} ;\left\{\mathrm{P}_{1}-\mathrm{P}_{2}=\rho \mathrm{gh}\right\}$