Which of the following complex is octahedral, diamagnetic and the...

The coordination compound ${K}_{3} [ Co ( CN{)}_{6} ]$contains a cobalt($III$) ion, which has a ${d}^{6}$ electronic configuration. The cyanide ligands in this complex are indeed strong field ligands, which means they will cause a large splitting of the $d$orbitals. So here cobalt is in $+ 3$ state having $3{d}^{6}$ configuration and also cyanide ligand which is a strong field ligand so, it is most stable and will have octahedral shape. The given compounds have the following nature:

(1) ${Na}_{3} [ {CoCl}_{6} ]$ – Paramagnetic

(2) \left[Ni{\left({NH}_{3}\right)}_{6}\right]{Cl}_{2} – Paramagnetic

(3) ${K}_{3}\left[Co ( CN{)}_{6}\right]$– Diamagnetic

(4) \left[Co{\left({H}_{2}O\right)}_{6}\right]{Cl}_{2} – Paramagnetic