Step 1: Introduction to Planting Systems
Planting systems are selected considering factors such as topography, slope, and soil characteristics.
Step 2: Hillside Planting Method
For hilly areas, planting is performed perpendicular to the slope (contour planting) to mitigate soil erosion and enhance moisture retention.
Step 3: Exclusion of Alternative Systems
- Square and rectangular planting methods are appropriate for flat terrains. - The hexagonal system is also employed on plains to maximize land usage. - Only the contour system is suitable for hilly regions.
Step 4: Determination
Consequently, the contour system is the preferred method for hills.
Final Answer: \[\boxed{\text{Fruit orchards on hills are planted using the Contour system.}}\]