The sentence that includes an unclear pronoun is:
Jamal and his cousin left at the same time, but she reached the park before he did.
In this sentence, it's unclear who "she" refers to since both Jamal and his cousin are mentioned. To correct it, you could specify who "she" is (e.g., "Jamal and his cousin left at the same time, but Jamal's cousin reached the park before he did.").