The option that always requires a comma before the conjunction is compound sentence.
In a compound sentence, when two independent clauses are joined by a coordinating conjunction (such as "and," "but," "or," etc.), a comma is usually placed before the conjunction.
For example:
- "I wanted to go for a walk, but it started to rain."
While items in a series, long introductory elements, and appositives have specific rules that may or may not involve commas, the need for a comma in compound sentences before the conjunction is a consistent rule.