Which steep hill percentage indicates the steepest grade?

Answer: D — 18%

The steepest grade of a hill is indicated by the highest percentage of incline. A percentage grade is calculated based on the rise over run, which means that a higher percentage corresponds to a steeper incline. In this context, an 18% grade means that for every 100 units of horizontal distance (run), the elevation changes by 18 units (rise), representing a very steep incline. As the percentage increases from 10% to 12% and then to 14%, and ultimately to 18%, it shows an increasing steepness. Therefore, an 18% grade is significantly steeper than the other options provided, making it the correct answer as it represents the most challenging incline for vehicles, particularly those operating with air brakes, which require careful management of speed and braking on such steep grades.