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. (Error: "who's" should be "whose" because it is possessive, not a contraction.)
-
Don't forget to pack your coat and hat, because its going to be snowing in Minnesota all week. (Error: "its" should be "it's" because it is a contraction of "it is," not a possessive.)
So the correct answers are the second and fifth sentences.