Final answer:
The sentence with the correct punctuation is: 'You need to study for the final; if you don't, you will probably not do well.'
Step-by-step explanation:
The sentence with the correct punctuation is: You need to study for the final; if you don't, you will probably not do well. This sentence correctly uses a semicolon to separate two related independent clauses. The semicolon shows that these clauses are closely connected.
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