Final answer:
The correct sentence is option C: Key West, the southernmost city in the United States, is located in Florida.
Step-by-step explanation:
The correct sentence that fixes the punctuation in the given sentence is option C: Key West, the southernmost city in the United States, is located in Florida. The original sentence lacks commas to set off the appositive phrase 'the southernmost city in the United States'. By adding commas before and after the appositive phrase, the sentence becomes grammatically correct and clearer to read.
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