Explanation

(d)
Strongest Bronsted base is that which has weakest conjugate acid.
[table] Base | Conjugate acid (base + $H^{+}$) ; ${ClO}^{-}$ | $HClO$ ; $ClO_{2}^{-}$ | $HClO_{2}$ ; $ClO_{3}^{-}$ | $HClO_{3}$ ; $ClO_{4}^{-}$ | $HClO_{4}$ [/table]
$\because$ $HClO$ is weak conjugate acid.
$\because$ $ClO^{-}$ is strongest Bronsted base.