b
Pure Solvent : \(C _{6} H _{6}(\ell)\)

Given : \(T _{ f }^{\circ}=5.5^{\circ} C\)

\(K _{ f }=5.12^{\circ}\,C / m\)

\(\because \Delta T _{ f }= k _{ f } \times m\)

\(\Rightarrow\left( T _{ f }^{0}- T _{ f }^{\prime}\right)=5.12 \times \frac{\left(\frac{10}{58}\right)}{\left(\frac{200}{1000}\right) \,kg } \,mol\)

\(\Rightarrow 5.5- T _{ f }^{\prime}=\frac{5.12 \times 5 \times 10}{58}\)

\(\Rightarrow T _{ f }^{\prime}=1.086^{\circ} C \simeq 1^{\circ} C\)