The correct answer is "I think you left your biology notebook on the bleachers yesterday"
Explanation:
The words "your" and "you're" are homophones because their pronunciation is the same and they are written similarly. However, they have a different meaning because the word "your" shows possession, while the word "you're" (you are) comprises a subject and the verb to be. In this context, the only sentence that is completely correct and uses these homophones correctly is "I think you left your biology notebook on the bleachers yesterday" because in this the word "your" is used to show possession, which matches its meaning.
