Correct Answer - Option 3 : 125 l/s

__Concept:__

The rate of flow from Venturimeter is given as

\(Q = \;{C_d}\frac{{{A_1}{A_2}}}{{\sqrt {A_1^2 - A_2^2} }} \times \sqrt {2gh} \)

Where,

A_{1} and A_{2} are cross sectional area at inlet and throat section

h = differential manometer head

\(h = X\;\left( {\;\frac{{{\rho _{Hg}}}}{\rho } - 1} \right)\)

Where,

X = differential manometer reading

__Calculation:__

Given,

d_{1} = 30 cm ; d_{2} = 15 cm, C_{d} = 0.98, x = 20 cm

\({A_1} = \frac{\pi }{4}{\left( {0.3} \right)^2} = 0.07065\;{m^2}\)

\({A_2} = \frac{\pi }{4}{\left( {.15} \right)^2} = .01766\;{m^2}\)

\(h = 0.2 \times \left( {\frac{{13.6}}{1} - 1} \right) = 2.52\;m\)

\(Q = 0.98 \times \frac{{.07065\; \times .\;01766}}{{\sqrt {{{\left( {.07065} \right)}^2} - {{\left( {01766} \right)}^2}} }} \times \sqrt {2 \times 9.81 \times 2.52} \)

Q = 128 l/s

Q ≈ 125 l/s