c
From lens Maker's formula

\(\frac{1}{f}=(\mu-1)\left(\frac{1}{R_{1}}-\frac{1}{R_{2}}\right)\)

If \(f_{1}, f_{2}\) are focal lengths of two plano convex lenses, then 

\(\therefore \frac{1}{f_{1}}=(1.5-1)\left(\frac{1}{20}-\frac{1}{\infty}\right)\)

\(\frac{1}{{{f_1}}} = (0.5)\left( {\frac{1}{{20}}} \right) = \frac{1}{{40}}\) \(\left[ {{\text{As}}\,\,{R_2}{\text{ is }}\infty } \right]\)

and \(\frac{1}{f_{2}}=(1.5-1)\left(\frac{1}{20}-\frac{1}{\infty}\right)=\frac{1}{40}\)

For concave lens of oil,

\(\frac{1}{{{f_3}}} = (1.7 - 1)\left( {\frac{{ - 1}}{{20}} + \frac{{ - 1}}{{20}}} \right)\) \( = 0.7 \times \frac{{ - 2}}{{20}} = \frac{{ - 7}}{{100}}\)

focal length of the combination is given by, 

\(\frac{1}{f}=\frac{1}{f_{1}}+\frac{1}{f_{2}}+\frac{1}{f_{3}}\)

\(\frac{1}{f}=\frac{1}{40}+\frac{1}{40}+\left(\frac{-7}{100}\right)\)

\(=\frac{5+5-14}{200}=\frac{-4}{200}=\frac{-1}{50}\)

\(f=-50\, \mathrm{cm}\)