d
\(\left[ Co ( CN )_{6}\right]^{3-}\) contains \(CO ^{3+}\) as central metal ion.

The electronic configuration of \(Co ^{3+}\) ion

\(=[A r]_{18} 3 d^{6}\)

The splitting in octahedral field is shown below

\(C o^{3+}=3 d^{6}\left(\right.\) or \(\left.t_{2 g}^{6} e_{g}\right)\)

\(\therefore\) There is no unpaired electron, so it is diamagnetic in nature.