Assertion: $[\mathrm{Ni}(\mathrm{CN}){}_4]{}^{2-}$ is diamagnetic whereas $[\mathrm{NiCl}{}_4]{}^{2-}$ is paramagnetic. Reason: The strong-field $\mathrm{CN}{}^{-}$ forces the $3d^8$ electrons of $\mathrm{Ni}{}^{2+}$ to pair up and gives $ds{p}^2$ hybridisation, while the weak-field $\mathrm{Cl}{}^{-}$ leaves them unpaired in an $s{p}^3$ complex.

1. ABoth Assertion and Reason are true, and the Reason is the correct explanation of the Assertion
2. BBoth Assertion and Reason are true, but the Reason is not the correct explanation of the Assertion
3. CAssertion is true but Reason is false
4. DAssertion is false but Reason is true