C. Both of these
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.
The slippery slope fallacy is something that many people fall into while writing something persuasive. This fallacy mostly happens when it is a primarily emotional argument with no factual support. The way it works is you make a claim that isn't entirely true, and then you base the rest of your argument on this point, even more "facts" off of this one slightly true fact you have stated. An example of this would be: "We can't allow people to go treasure hunting. By going treasure hunting people are more likely to trespass on other people's land in order to find treasure. By making trespassing less serious, people will start to loss their sense of other people's privacy. As you can see, if we allow people to pursuer treasure hunting, they will eventually start to break into other people's homes."
The answer you are looking for is B.)