When should you use a comma?

Using a comma to separate elements in a series is a fundamental rule in English grammar. This practice helps clarify meaning by distinguishing between different items within a list, making it easier for the reader to understand the information being presented. For example, in the phrase "apples, oranges, and bananas," the commas clearly separate the different types of fruit, indicating that they are distinct items.

In contrast, the other choices do not align with the correct use of commas. Commas are not used to end sentences or to precede periods; rather, periods signal the end of a sentence. Additionally, while it is sometimes appropriate to use a comma after certain conjunctions, this is not a defining rule and does not represent the primary use of commas in the English language. Thus, the choice to use a comma to separate elements in a series stands out as a clear and correct application of this punctuation mark.