Elevation in boiling point ($\Delta T_b$) is a colligative property, which means it depends on the total number of solute particles in the solution . For electrolytes, $\Delta T_b = i \times K_b \times m$, where $i$ is the van't Hoff factor. For dilute aqueous solutions, molarity ($M$) is approximately equal to molality ($m$).

Let's calculate the effective concentration of particles ($i \times M$) assuming complete dissociation:

1. 0.05 M NaCl: Dissociates into $Na^+$ and $Cl^-$, so $i=2$. Effective concentration = $2 \times 0.05 = 0.10$ M.
2. 0.1 M KCl: Dissociates into $K^+$ and $Cl^-$, so $i=2$. Effective concentration = $2 \times 0.1 = 0.20$ M.
3. 0.1 M $MgSO_4$: Dissociates into $Mg^{2+}$ and $SO_4^{2-}$, so $i=2$. Effective concentration = $2 \times 0.1 = 0.20$ M.
4. 0.1 M NaCl: Dissociates into $Na^+$ and $Cl^-$, so $i=2$. Effective concentration = $2 \times 0.1 = 0.20$ M.

Since 0.05 M NaCl has the lowest effective particle concentration (0.10 M), it will exhibit the least elevation in boiling point.