I've been having a really hard time with this one as well. At first, I was going to say option three (acronym) because the root part of it made me think more of something like sigil, or rune, and an acronym seemed more like a runic way of putting something across. Plus, it was different from the others, and it seemed as though it could be a tricky answer.
But then I thought about it some more, and realized that "signal" and sign" could really be just as runic at heart, and had the plus of a similar spelling. Plus, "sign" and "signal" can sometimes be synonymous anyway.
Decisions, decisions...why would you have put in a word like "acronym" with no visual similarity if it didn't have anything to do with it?
So I'm going to just go with all three.