Answer: 0.5 M ionic 
Explanation: Addition of nonvolatile solute depresses the freezing point of a solvent. As depression in freezing point is a colligative property, which means it depends on the amount of solute. Thus more is the no of ions, more is the depression in freezing point.

=Depression in freezing point
= freezing point constant
m= molality
a) pure water: It is a pure solvent and freezes at 0 ° C.
b) 0.5M ionic NaCl:
, it produces two ions, each of concentration 0.5M. The total concentration of ions is 1.0 M.
c) 0.5 M ionic 
, it produces two ions,
of concentration 0.5M and
of concentration
. The total concentration of ions is 1.5 M.
d) 0.5 M ionic 
, it produces two ions,
of concentration 0.5 M and
of concentration
The total concentration of ions is 2.0 M.
e) 0.5 M molecular 
It is a covalent compound and do not dissociate into ions. thus the concentration will be 0.5 M only.
As the concentration of ions is highest in 0.5 M ionic
, the depression in freezing point is highest ans thus it has lowest freezing point.