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The correct response identifies CL as the best aquitard according to the Unified Soil Classification System. Aquitards are geological formations that, while not completely impermeable, significantly restrict the flow of water. They are typically characterized by finer-grained materials that have lower permeability compared to coarser materials.

CL, which represents clay with a low plasticity, has a small particle size and a structured arrangement that allows it to retain moisture and limit the movement of water through it. This property makes CL an effective aquitard because its capacity to restrict water flow can help protect aquifers by serving as a barrier to contaminants or preventing the rapid drainage of aquifer water.

In contrast, the other soil types listed have higher permeability and would not serve well as aquitards. SP represents poorly graded sand, which allows water to flow relatively freely, and would thus not effectively restrict water movement. ML denotes silt with low plasticity, which, while finer than sand, does not have the effective sealing capability of clay. SW indicates well-graded sand and gravel, which is highly permeable and unsuitable for aquitard purposes.

Understanding these classifications allows for the appropriate selection of soil types in engineering and environmental contexts, particularly in the management of groundwater