The option that always requires a comma before the conjunction is compound sentence.
In a compound sentence, when you connect two independent clauses with a coordinating conjunction (such as "and," "but," "or," etc.), you should place a comma before the conjunction.
For example:
- I wanted to go for a walk, but it started to rain.
The other options may require commas in certain situations, but they do not always require a comma before a conjunction.