The option that always requires a comma before the conjunction is d. compound sentences.
In a compound sentence, if you are joining two independent clauses with a conjunction (such as "and," "but," or "or"), you generally place a comma before the conjunction.
Here’s a brief explanation of the other options:
- a. long introductory element: A comma is typically used after a long introductory phrase, but it’s based on length or clarity rather than always being required.
- b. appositive: Commas are used in appositives, but they are not always required, especially in restrictive appositives.
- c. items in a series: A comma is used to separate items in a series, but the use of the Oxford comma (the comma before the final "and" or "or") is optional based on style.
So, option d is your best choice.