Let $\mathit{X}\mathit{g}$ of the compound is to be dissolved. Number of moles of compound,

Mass of Benzene $=75\mathit{g}=0.075\mathit{K}\mathit{g}$
Molality
$=\frac{\mathit{W}}{256\times 0.075}=\frac{\mathit{W}}{19.2}$
Freezing point depression $\mathrm{\Delta }{\mathit{T}}_{\mathit{f}}=0.48\mathrm{}\mathit{K}$
Molar depression of freezing point constant $=5.12\mathrm{}\mathit{K}\mathrm{}\mathit{k}\mathit{g}∕\mathrm{}\mathit{m}\mathit{o}\mathit{l}$
It is defined as the total number of molecules of reactants.
$\Rightarrow {\mathit{T}}_{\mathit{f}}={\mathit{K}}_{\mathit{f}}0.48$
$=5.12\mathrm{}\mathit{W}=1.8\mathrm{}\mathit{g}$
Hence, $1.8\mathrm{}\mathit{g}$ solute has been dissolved.