c
\(\begin{gathered}
{\text{[O}}{{\text{H}}^{\text{ - }}}] = \sqrt {{K_b} \times C} \hfill \\
= \sqrt {{{10}^{ - 5}} \times {{10}^{ - 3}}} = \sqrt {{{10}^{ - 8}}} = {10^{ - 4}} \hfill \\
\end{gathered} \)
\(pOH\) =\( -log [OH^{-}]\) = \(- log [10^{-4}] \) = \( 4\, log\, 10 = 4\)