What do index properties include for soil testing?

Index properties are fundamental characteristics of soils that provide insights into their behavior and suitability for engineering purposes. The correct choice reflects that moisture content, grain size, and plasticity are all essential index properties.

Moisture content is critical because it influences the soil's strength, compressibility, and overall behavior under different conditions. Grain size distribution gives an understanding of how soil particles are categorized, which plays a role in drainage and compaction characteristics. Plasticity is important as it indicates the soil's ability to deform without cracking, influencing stability and construction practices.

Each of these properties contributes to a comprehensive understanding of soil behavior, making them vital for assessing soil quality and suitability for various engineering applications. In contrast, focusing only on moisture content or grain size lacks the breadth needed to evaluate the soil comprehensively, as they ignore significant characteristics that affect performance and usability.