which aqueous solution has minimum freezing point ?

(a)0.01M NaCl

(b) 0.005M C₂H₅OH

(c) 0.005M MgI₂

(d) 0.005M MgSO₄

The simple inspection of the question show that (c) should be the correct choice because MgI₂ will give 3 mol of ions for 1 mol of MgI₂. However, as the given concentration of solute is more in case of NaCl i.e. 0.01M than MgI₂ i.e. 0.005M. Therefore NaCl solution will have minimum freezing point.
Option (a) is correct.