The correct option is B 1.73 BM
In [Fe(CN)6]3−, the oxidation state of Fe is +3 and the configuration is [Ar]3d5. As CN− is a strong field ligand, there will be the pairing of electrons and only 1 electron will be unpaired.
∴ n=1
μspin=√n(n+2)=√1(1+2)
=√3=1.732=1.73 BM