Answer
See the answer below.
Work Step by Step
By definition, the freezing point depression is $\Delta T_f=K_f.m.i$ Since these are all aqueous solutions, the ones with the lowest freezing points are the ones with the greater $i.m$ product, and we need to assume complete ionic dissociation, i.e, i is the number of ions per compound:
a) $i.m=1\times 0.20m=0.20m$
b) $i.m=3\times 0.12\ m=0.36\ m$
c) $i.m=3\times 0.10\ m=0.30\ m$
d) $i.m=2\times 0.12\ m=0.24\ m$
$a\gt d\gt c\gt b$
