The two sentences that include errors in the use of possessives or contractions are:
-
"We are not certain who's textbooks these are, but we want to find out so that we can return them."
- Correction: "who's" should be "whose."
-
"Don't forget to pack your coat and hat, because its going to be snowing in Minnesota all week."
- Correction: "its" should be "it's."
So the correct answers are the second and fourth sentences.