Before driving, you must:

Answer: B — Put on a seatbelt

Before driving, it is important to put on your seatbelt for your safety. While options A, C, and D may also be important tasks to do before driving, they are not as crucial to your safety as putting on a seatbelt. Checking the brakes and mirrors can be done once inside the car, and getting the registration papers can be done after you have started your journey. Therefore, option B, putting on a seatbelt, is the most important task to do before driving.