c
$\frac{P}{Q} = \frac{R}{{S'}}$ (For balancing bridge)
$ \Rightarrow $ $S' = \frac{{4 \times 11}}{9} = \frac{{44}}{9}$
$ \Rightarrow $ $\frac{1}{{S'}} = \frac{1}{r} + \frac{1}{6}$
$ \Rightarrow $ $\frac{9}{{44}} - \frac{1}{6} = \frac{1}{r}$
$ \Rightarrow $ $r = \frac{{132}}{5} = 26.4\,\Omega $
