I haven't started Unit D yet but should soon. That being said, I did complete the Professional Organization course and I had to practice floorplans exactly like what you find in Assignment D5. I don't remember receiving a pen with the course material but I remember purchasing a pack of black art pens from Amazon. They were the following: "Precision Black Micro-Pen Fineliner Ink Pens, Set of 9" but I think you can get any black art pen as long as it make your lines appear sharp and clean. I like the package because you have different sizes and depending on the amount of pressure you put on your pencil, it will determine the size of pen that can cover your lines. If you do purchase just 1 pen, just make sure it's not too thin, so make sure it will be preferably the same size as your pencil lines at a minimum but not too big either.
I hope this helps