A well screen must be installed to serve which type of materials?

The installation of a well screen is essential for wells that are constructed in unconsolidated materials, such as sand, gravel, or silt. These materials are loose and require a well screen to prevent the finer particles from entering the well while still allowing groundwater to flow into it. The well screen serves as a barrier that filters out sediments, helping to maintain water quality and ensuring that the well operates efficiently.

In contrast, solid rock does not require a screen the same way unconsolidated materials do because water typically seeps through fractures or fissures in the rock, and there isn't a risk of fine particulates entering the well. Clay formations are often impermeable, meaning they don't allow water to flow through them easily and generally don’t necessitate a well screen for their extraction. Aquifers, being water-bearing formations, may contain both consolidated and unconsolidated materials, and the need for a well screen would depend on the specific nature of the material where the well is drilled. Therefore, the correct choice highlights the necessity of well screens primarily in unconsolidated materials, which need such filtration to provide clean, drinkable water.