# Critical frequency $\nu_c$ for ionsphere is

$\begin{array}{1 1}(a)\;9N_{max}\\(b)\; 9\sqrt{N_{max}}\\(c)\;3\sqrt{N_{max}}\\(d)\;6N_{max}\end{array}$

Critical frequency $\nu_c$ for ionsphere is $9\sqrt{N_{max}}$
Hence (b) is the correct answer.