The option that always requires a comma before the conjunction is compound sentence.
In a compound sentence, when you join two independent clauses with a conjunction (like "and," "but," or "or"), a comma is placed before the conjunction.
For example:
- "I wanted to go for a walk, but it started to rain."
The other options (appositive, items in a series, and long introductory element) have specific rules regarding comma usage but don't always require a comma before the conjunction.