What should you do before backing up with a multi-trailer configuration?

Answer: A — Check mirrors and blind spots

Before backing up with a multi-trailer configuration, it is crucial to check mirrors and blind spots. This practice ensures that you have a clear view of your surroundings and can identify any obstacles or other vehicles that may be in your path. Given the length and maneuverability challenges associated with multi-trailer setups, relying solely on mirrors is essential, as they help provide the widest possible perspective of the area behind you. Proper backing procedures are fundamental to safety, especially when handling larger vehicles that can have significant blind spots. Ensuring awareness of your environment allows for safer navigation and prevents potential collisions or accidents. Therefore, checking mirrors and blind spots is not just a recommended step; it's a necessary one to operate effectively and responsibly. The other options do not address the importance of situational awareness when reversing with a large configuration, making them less relevant in this context.