With what neutral molecule is $ClO^{-}$ isoelectronic? Is that molecule a Lewis base?

(B) $ClO^{-}$ is isoelectronic to $ClF$. Both species contain $26$ electrons in total.
Total electrons in $ClO^{-} = 17 + 8 + 1 = 26$.
Total electrons in $ClF = 17 + 9 = 26$.
$ClF$ acts as a Lewis base because it has lone pairs of electrons that can be donated to form compounds like $ClF_{3}$.