Correct Answer - Option 4 : 64/Re

__Explanation:__

Darcy Weisbach Equation for friction losses in circular pipe:

\({h_f} = \frac{{f \times L \times {V^2}}}{{2 \times g \times D}}\)

where,

L = length of the pipe, D = diameter of the circular pipe, V = mean velocity of the flow, f = Darcy’s friction factor = 4 × F’, F’ = coefficient of friction, hf = head loss due to friction

For Laminar Flow

Friction Factor \(f = \frac{{64}}{{{R_e}}}\;and\;F' = \frac{1}{4} \times \frac{{64}}{{{R_e}}} = \frac{{16}}{{{R_e}}}\)

For Turbulent flow

\(f = \frac{{0.316}}{{R_e^{\frac{1}{4}}}}\)