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, the pronoun "she" is unclear because it could refer to either Jamal or his cousin without additional context clarifying who "she" is. To correct it, one could specify which person reached the park first, for example: "Jamal and his cousin left at the same time, but Jamal's cousin reached the park before he did."