Correct Answer - Option 3 : 0.5

__Concept:__

According to the Blasius equation, for a laminar flow

Boundary layer thickness (δ) can be calculated by:

\(\delta = \frac{{5x}}{{\sqrt {R{e_x}} }}\)

where Re_{x} = Reynold's number = \(\frac{{\rho Vx}}{\mu }\) {V = Velocity}

\(\therefore \delta \propto \frac{1}{V}\)

__Calculation:__

__Given:__

δ_{1} = 1 mm, V_{2} = 4V_{1}

\(As,\,\delta \propto \frac{1}{V}\)

\(\frac{{{\delta _1}}}{{{\delta _2}}} = \sqrt {\frac{{{V_2}}}{{{V_1}}}} \)

\(\frac{1}{{{\delta _2}}} = \sqrt {\frac{4}{1}} = 2\)

δ_{2} = **0.5 mm**