$(a)\;2R \\ (b)\;\frac{R}{2} \\ (c)\;6R \\ (d)\;\frac{R}{6} $

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For a Plano convex lens

$\large\frac{1}{f_1}=(\mu_1-1) \bigg[ \large\frac{1}{\infty} -\frac{1}{-R}\bigg]$

$\large\frac{1}{f_2}=(\mu_2-1) \bigg[ \large\frac{1}{-R} -\frac{1}{\infty}\bigg]$

For the combination of lens

$\large\frac{1}{F}=\frac{1}{f_1} +\frac{1}{f_2}$

$\qquad= \large\frac{\mu_1 -\mu_2}{R}$

$F= \large\frac{R}{\mu_1 -\mu_2}$

$\quad= \large\frac{R}{\Large\frac{3}{2}-\frac{4}{3} }$

$\quad = \large\frac{R}{\large\frac{1}{6} }$

$F= 6R$

Hence c is the correct answer.

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