Answer
(d) $CN^-$ is the strongest conjugate base.
Work Step by Step
The strength of the acid is inversely proportional to the strength of the conjugate base: The stronger the acid, the weaker its conjugate base.
In order to identify the strongest conjugate base, we must find the weakest acid, which is the one with lowest $K_a$.
The acid with lowest $K_a$ in this table is $HCN$; therefore, $CN^-$ is the strongest conjugate base.
