Does completing a B98/B99 guarantee that I can do a C or D-level research course with my supervisor?

No. Your continuing research in a lab is entirely up to both you and your supervisor. While completing a B98 or B99 does not guarantee you a spot in a C or D-level research course, it can be your chance to formulate a compelling independent research project and convince your supervisor that you are a talented, hard-working student deserving of that opportunity!