The correct answer is b. to show a pause in a sentence or separate items in a list.
Commas are used in several contexts, including:
- Separating items in a list (e.g., "I bought apples, oranges, and bananas.")
- Indicating a pause in a sentence (e.g., "After dinner, we went for a walk.")
- Setting off introductory elements (e.g., "However, I still believe it is possible.")
- Separating clauses (e.g., "I would go, but I have a prior engagement.")
While options c and d describe some specific uses of punctuation (like dashes for abrupt breaks or ellipses for omissions), they do not accurately describe the primary function of commas.