Assertion : Potassium ferrocyanide is diamagnetic whereas potassium ferricyanide is paramagnetic.Reason : Crystal field splitting in ferrocyanide ion is greater than that of ferricyanide ion.

In ferrocyanide $[\text{Fe}(\text{CN}{)}_6{]}^{4-}$, $\text{Fe}$ is in $+2$ state ($d^6$) and strong field ligands cause all electrons to pair up (diamagnetic). In ferricyanide $[\text{Fe}(\text{CN}{)}_6{]}^{3-}$, $\text{Fe}$ is in $+3$ state ($d^5$), resulting in one unpaired electron (paramagnetic). Higher oxidation states generally lead to larger crystal field splitting. The assertion is true, but the reason is false as ferricyanide has greater splitting than ferrocyanide.