What must you do to prevent a two-axle truck from rolling down a steep slope?

Your answer is correct. To prevent a two-axle truck from rolling down a steep slope, you must engage the parking brake and place a wheel chock under a rear wheel. The parking brake holds the vehicle stationary, while the wheel chock provides additional safety by physically blocking the wheel, especially important on steep inclines. Simply using the retarders is not sufficient for parking, as they are designed for controlling speed during driving, not for securing a parked vehicle. Combining the parking brake with wheel chocks ensures the truck stays safely in place.