Oil of Mirbane on reduction with ${\mathrm{LiAlH}}_4$ gives orange red dye (Azobenzene). In this reaction, the equivalent weight of nitrobenzene is $[M=Molecular weight ]$

Oil of mirbane is chemically known as nitrobenzene.

Now, reduction of nitrobenzene to azobenzene,

Equivalent weight $=\frac{\text{ Mol.wt }}{\mathrm{n}-\text{ factor }}$

$=\frac{\mathrm{M}}{4}$

Hence, option (B) is correct.