c
(c)If two drops of same radius r coalesce then radius of new drop is given by R
\(\frac{4}{3}\pi {R^3} = \frac{4}{3}\pi {r^3} + \frac{4}{3}\pi {r^3}\)==> \({R^3} = 2{r^3} \Rightarrow R = {2^{1/3}}r\)
If drop of radius r is falling in viscous medium then it acquire a critical velocity v and \(v \propto {r^2}\)
\(\frac{{{v_2}}}{{{v_1}}} = {\left( {\frac{R}{r}} \right)^2} = {\left( {\frac{{{2^{1/3}}r}}{r}} \right)^2}\)
==> \({v_2} = {2^{2/3}} \times {v_1} = {2^{2/3}} \times (5) = 5 \times {(4)^{1/3}}m/s\)