Which of the following sentences correctly uses a comma?

Using a comma correctly helps clarify meaning and improve the flow of a sentence. In the example provided, the sentence "Before dinner I need to finish my homework" correctly places a comma at the beginning, separating the introductory phrase "Before dinner" from the main clause. This separation makes it easier for the reader to comprehend the temporal context of the action being described.

In contrast, other sentences represented do not correctly utilize commas. For example, "John likes to swim run and bike" lacks a comma to separate the items in a list, which leads to confusion about the items being listed. The sentence "I enjoy skiing, hiking, and to swim" improperly mixes parallel structures since "to swim" does not match the gerund form of the other activities. The final option, "My friends, Anna and Mike are coming over," incorrectly places the commas around "Anna and Mike," which are essential to the sentence and should not be set off with commas since they are integral to the meaning of "my friends."

Hence, the well-placed comma in option B distinctly enhances clarity and maintains grammatical correctness.