Final answer:
In the sentence 'I really could use some help with this one, if you have the time for it', the use of a comma is correct without incorporating any conjunction. The comma divides the sentence into a main clause and a dependent clause, which correctly adheres to one of the predefined rules of comma usage.
Step-by-step explanation:
In English grammar, the use of commas is often associated with conjunctions. However, they could be correctly used without conjunctions. For instance, you observed four sentences and asked which one uses a comma correctly without a conjunction. The correctly punctuated sentence is: 'I really could use some help with this one, if you have the time for it.' The comma splits the sentence into two distinct but related ideas. It separates the main clause ('I really could use some help with this one') from the dependent clause ('if you have the time for it'). This correctly follows one of the rules of comma usage where it separates a dependent clause beginning with 'if' from the rest of the sentence.
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