Question

Compound sentences are incomplete if they are missing a

Answer 1

A. Comma.
If a compound sentence does not have a comma it is in complete.

Answer 2

A. Comma. Every compound sentence is two simple sentences, joined by a conjunction and a comma. Like this: My day was really great, but I fell and skinned my knees. This is a compound sentence because you could split it into 2, without the comma or the conjunction, like so: My day was really great. I fell and skinned my knees.