ANS:D - unified soil classification system.

For general engineering purposes, soils are commonly classified using the Unified Soil Classification System (USCS). The Unified Soil Classification System is a widely used method for classifying soils based on their physical properties, including particle size distribution, plasticity, and other engineering characteristics. It categorizes soils into various groups based on their grain size, plasticity, and other engineering properties, providing engineers with valuable information for designing and constructing structures, foundations, and earthworks.