A signal statement tells people that something important is coming so the listener or reader needs to pay close attention. The statement "keep in mind" signals to the reader that what comes next could be an important factor. The second statement "I want you to understand..." also signals to the reader or listener that what will follow is important for the reader to pay attention to.
I think the answer is c. It could be both because they are both signaling that the other person shouldn't forget something that could change their decision and/ or saying I want you to understand because they're about to tell them something bad.
