c
To obtain final image at infinity, object which is the image formed by objective should be at focal distance of eye-piece. By lens formula (for objective)

\(\frac{1}{v_{0}}-\frac{1}{u_{0}}=\frac{1}{f_{0}}\)

\(\alpha, \quad \frac{1}{v_{0}}-\frac{1}{-25}=\frac{1}{20}\)

\(\Rightarrow \quad \frac{1}{v_{0}}=\frac{1}{20}-\frac{1}{25}=\frac{5-4}{100}=\frac{1}{100}\,\mathrm{mm}\)

\(\therefore \quad v_{0}=100\, \mathrm{mm}\)

Therefore the distance between the lenses

\(=v_{0}+f_{e}=100\, \mathrm{mm}+20\, \mathrm{mm}=120\, \mathrm{mm}\)