d
(d) $n = \frac{1}{{2l}}\sqrt {\frac{T}{{\pi {r^2}\rho }}}$
$\Rightarrow n \propto \frac{{\sqrt T }}{{lr}}$
==> $\frac{{{n_1}}}{{{n_2}}} = \sqrt {\frac{{{T_1}}}{{{T_2}}}} \times \frac{{{l_2}}}{{{l_1}}} \times \frac{{{r_2}}}{{{r_1}}}$
$ = \sqrt {\frac{T}{{3T}}} \times \frac{{3l}}{l} \times \frac{{2r}}{r} = 3\sqrt 3 $
==> ${n_2} = \frac{n}{{3\sqrt 3 }}$