Boiling point is least for

 We know that Elevation of the Boiling point=

$\to △{\mathrm{T}}_{\mathrm{b}}\propto \mathrm{m}$

$\to △{\mathrm{T}}_{\mathrm{b}}={\mathrm{k}}_{\mathrm{b}}\mathrm{m}$

As we can see Elevation of the Boiling point is directly proportional to molality.

So, $\mathrm{NaCl}$ and ${\mathrm{MgCl}}_2$ are ionic solid so they have more boiling point than non-ionic solid.

Therefore, $0.1 \mathrm{m}$ $\mathrm{urea}$ has the least boiling point as it has least molal concentration.

Hence, option(a) is correct.