All those compounds are ionic.
Then they dissociate into ions when they are in water.
I will write the dissociation equations. The charges of the ions is indicated inside parenthesis.
KI -> K (+) + I (-) => two ions
K2SO4 -> 2 k(+) + SO4 (2-) -> three ions
NaCl -> Na(+) + Cl(-) => two ions
KBr -> K(+) + Br(-)
As you se every molecule of the compound K2SO4 dissociates into three ions, while the other compounds dissociate into two ions. Given that the decrase in the freezong point of water is proportional to the number of particles dissolved, the K2SO4 will lower the freezing point the most.
Answer: option B. K2SO4
