Answer:
The sentence She thought she saw a stranger but he turned out to be her neighbor who was back from vacation early is a compound-complex sentence.
Step-by-step explanation:
A compound-complex sentence contains at least two independent clauses and at least one dependent clause. In this case, She thought she saw a stranger and he turned out to be her neighbour are the independent clauses and who was back from vacation early is a dependent clause. Therefore, this is a compound-complex sentence.