(c)
$\left\lbrack {OH}^{-} \right\rbrack = \sqrt{K_{b} \times C}$
$\ \ \ \ \ \ \ \ \ \ \ \ \ = \sqrt{1 \times 10^{- 5} \times 10^{- 1}}$
$= \sqrt{10^{- 6}} = 10^{- 3}$
$K_{w} = \left\lbrack H^{+} \right\rbrack\lbrack{OH}^{-}\rbrack$
$10^{- 14} = \left\lbrack H^{+} \right\rbrack\lbrack 10^{- 3}\rbrack$
$\left\lbrack H^{+} \right\rbrack = 10^{- 11}$
Hence, $pH = - \log H^{+}$
$= - \log{(1 \times 10^{- 11})} = 11$