Final answer:
A comma can correct most run-on sentences when it is paired with a coordinating conjunction to join two independent clauses. It is not correct to use a comma alone to separate two independent clauses or to join two dependent clauses without an introductory phrase or contrasting meaning.
Step-by-step explanation:
When correcting run-on sentences, a comma by itself is not sufficient. However, a comma can be used to connect two independent clauses if it is accompanied by a coordinating conjunction, such as 'for', 'and', 'nor', 'but', 'or', 'yet', or 'so'. These conjunctions are remembered using the acronym FANBOYS. In contrast, when two dependent clauses are joined, a comma is generally not required unless there's an introductory phrase or a contrast.
Therefore, the correct answer to the question about commas is:
d. Is required to combine two dependent clauses correctly is incorrect because dependent clauses typically do not require a comma to join them unless one is introducing the other.