What feature would you expect to find in the design of a drainage system to manage runoff?

A comprehensive drainage system designed to manage runoff incorporates multiple features that work together to effectively handle excess water. Water retention basins are crucial as they allow for the temporary storage of runoff, reducing the speed and volume that reaches downstream areas. This helps prevent flooding and erosion and can promote groundwater recharge.

Catchment areas are also fundamental as they define the geographical region from which runoff is collected. Properly delineating these areas ensures that the system is designed with adequate capacity to handle the expected volume of water.

Downspouts play an essential role in directing rainwater from rooftops down to the drainage system, ensuring that stormwater is channeled appropriately and reducing the risk of surface runoff that could lead to flooding.

By incorporating all these elements—water retention basins, catchment areas, and downspouts—the drainage system becomes more efficient in managing runoff, making option D the most comprehensive and correct answer.