The cobalt atom in $Hg \left[ Co ( SCN )_{4}\right]$ carries +2 charge and has $3 d ^{7}$ as the outer electronic configuration.
Here Hg shows +2 oxidation state and the compound is known as Mercuric tetrathiocyanatocobaltate(II).
It contains three unpaired electrons.
For three unpaired electrons, the expression becomes $\mu_{e f f}=\sqrt{3(3+2)} B \cdot M .=\sqrt{15} B \cdot M$
Therefore, option $C$ is correct.
(પ.ક્ર.: $Mn\, = 25, Co\, = 27, Ni\, = 28, Zn\, = 30$)