The 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: it should be "whose" instead of "who's.") -
Don't forget to pack your coat and hat, because its going to be snowing in Minnesota all week.
(Correction: it should be "it's" instead of "its.")
So, the two correct answers are the third and fifth sentences.