Which option describes the smallest particle size range in soil classification?

The option that describes the smallest particle size range in soil classification is the one indicating particles smaller than 0.075 mm. This classification refers to clay and silt particles, which are generally defined as having a very fine texture. In soil science, the size ranges for different types of soil particles are well established, and particles less than 0.075 mm are classified as silt and clay.

Clay, in particular, has significant engineering properties, such as plasticity, compressibility, and the ability to retain water, making it important for various civil engineering applications. Understanding this particle size range allows engineers to assess soil behavior, permeability, and suitability for construction projects.

In contrast, the other options indicate larger particle sizes: 4.75–75 mm refers to coarse aggregates like sand and gravel, while particles larger than 75 mm indicate boulders and larger aggregates. Therefore, the correct answer highlights the smallest particle size, which is crucial for understanding the finer aspects of soil behavior and classification in engineering practices.