"I'm worrying" is about how you're feeling right now.. "I worry" is just a general statement and usually just means that you have that feeling sometimes.
I'd like to note that realistically, B and D are very situational, as in it's almost never useful to point out that it's right now that you worry about something. The only use-case I can imagine is explanatory, e.g. if someone asks you why you're shaking. Even then, "worried" is more common.