All of the given options can sometimes be reasons for using a comma, but the most accurate and general answer would be options b and d: to create a pause and to change the subject. Commas are primarily used to indicate a pause in a sentence or to separate different parts of a sentence, such as clauses or items in a list. They can also be used to indicate a change in subject or to separate introductory phrases or interjections. Comma usage to build suspense or to end a sentence would be less common and situational.
