$\mu_n=\sqrt{\big(\large\frac{ze^2}{mr_n}\big)}$
Radius of $III$ orbit =$r_1\times n^2=0.529\times 10^{-8}\times 9cm$
$\mu_n=\sqrt{\big(\large\frac{1\times (4.083\times 10^{-10})^2}{9.108\times 10^{-28}\times 0.529\times 10^{-8}\times 9}\big)}$
$\mu_n=7.29\times 10^7cmsec^{-1}$
Hence (a) is the correct answer.