Final answer:
The correct sentence is 'You can turn right on a red light, but you can't turn left.'
Step-by-step explanation:
The correct sentence that uses the comma correctly is:
You can turn right on a red light, but you can't turn left.
Commas are used to separate independent clauses joined by coordinating conjunctions, like 'but'. In this sentence, the comma correctly separates the two independent clauses 'You can turn right on a red light' and 'you can't turn left'.
Learn more about Correct use of commas in sentences