Final answer:
Sentence 8 is the run-on sentence as it improperly joins two independent clauses without the required punctuation or conjunction.
Step-by-step explanation:
The sentence identified as a run-on sentence in the provided text is sentence 8. A run-on sentence occurs when two or more independent clauses are connected improperly. Sentence 8 compounds multiple ideas without the proper punctuation or coordinating conjunctions to separate them. It effectively joins two independent clauses: 'Fitzgerald would spend the majority of his career struggling to earn as much money as possible' and 'to maintain the privileged lifestyle that Zelda desired,' without a conjunction or proper punctuation. This makes it a run-on sentence, which grammatically requires revision for clarity.