B) Tell the Gundersons that if our lawnmower breaks, then we want to borrow their's.
Explanation:
The sentence that has a possessive noun error is the following one: Tell the Gundersons that if our lawnmower breaks, then we want to borrow their's. The word "theirs" never has an apostrophe to show possession. The word shows possession effectively without an apostrophe.
