What device is designed to secure the apparatus when loaded to its maximum in-service weight on a 10% grade?

The correct choice is wheel chocks, as they are specifically designed to prevent movement of a vehicle or apparatus when it is parked, especially when facing an incline. When a fire apparatus is loaded to its maximum in-service weight and positioned on a grade of 10%, the weight distribution can shift, increasing the risk of the vehicle rolling or sliding. Wheel chocks create a physical barrier against the wheels, effectively immobilizing the apparatus and improving safety for firefighters and equipment during operations.

Other devices like stabilizers are typically used to provide additional support and stability to the apparatus but may not be specifically tailored to secure against rolling. Fire hydrants are infrastructure used for water supply and have no relation to securing vehicles. Safety cones serve as visual warnings or to redirect traffic but do not provide physical stabilization for a parked vehicle. Thus, while all options have their respective uses, wheel chocks are the most appropriate and effective solution for securing an apparatus on a slope.