Answer:
D) The addition of different substances affects the freezing point of water.
Step-by-step explanation:
The best answer would be letter D.
The short explanation would be, the other choices only involves the addition of 1 substance (salt or sugar).
The long explanation:
Susie is testing two types of solution: sugar + water and salt + water. This means that she is testing the effect of both. An experiment is done to test certain variables and an addition of a variable that is not being tested is not advisable. But since she intentionally used both solutions, then it is reason enough to assume that she is testing both substances (sugar and salt) so the hypothesis should include both as well.