(a)
For non-electrolyte
$\mathrm{\Delta}T_{f} = k_{f} \times m$
Given, m =0.05,
$\therefore\ \ \ \ \ \ \ \ \ \ \ \mathrm{\Delta}T_{f} = 1.86 \times 0.05 = 0.093℃$
Freezing point of solution
$k_{f} = 1.86 = 0 - \mathrm{\Delta}T_{f}$
=0-0.093=-0.093$℃$