Correct Answer - C
From irignometry
`R^(2) = (3)^(2) + (R-0.3)^(2) implies R = 15 cm`
`mu_("lens") = (3 xx 10^(8))/(2 xx 10^(8)) = 1.5`
`therefore` from Lens maker formula ,
`(1)/(f) = ((mu_(L))/(mu_(s)) - 1) ((1)/(R_(1)) - (1)/(R_(2))) = 0.5 ((1)/(15)) = (1)/(30)`
f = 30 cm