Which of these solutions has the lowest freezing point?
0.25 m nacl
0.5 m nacl
1.0 m nacl
1.5 m nacl
2.0 m nacl

Answer is: 2.0 M NaCl.

Change in freezing point from pure solvent (water) to solution (sodium chloride solution): ΔT = i · Kf · c.

Kf - molal freezing-point depression constant for water is 1.86°C/m.

c-  molarity of the solution.

i - Van't Hoff factor.  

Because molal freezing-point depression constant and Van't Hoff factor are the same for all five solutions, freezing point depends on molarity of the solution.

The higher is the molarity, the lower is freezing point.

The temperature at which the phase transition solid-liquid occurs is the melting point or the freezing point.