Answer:
Option B
Explanation:
When S is donor atom of SCN- , it produces weak ligand field and forms high spin complex as
[Fe(SCN)6]3−;Fe3+(3d5)=

Spin only magnetic moment
(μs)=√5(5+2)BM=√35BM
In case of CN- ligand , carbon is the donor atom, it produces strong field and forms low spin complex as
[Fe(CN)6]3−:Fe3+(3d5)

Spin only magnetic moment (μs)=√1(1+2)BM
=√3BM
Hence, difference in spin only magnetic moment
=√35−√3=4BM