Particle-size limits: According to standard soil classification, the maximum size for particles classified as clay is:

Introduction / Context:
Soil classification systems (such as IS, BS, and ASTM) define particle-size boundaries for clays, silts, sands, and gravels. These thresholds underpin laboratory testing, specification compliance, and interpretation of index properties like plasticity and activity.

Given Data / Assumptions:

• “Clay-size” refers to particle-size limits used in standard classifications.
• We are concerned with the upper boundary for clay-sized particles.

Concept / Approach:
Clay-size particles are those finer than 0.002 mm. Particles between about 0.002 mm and 0.075 mm are typically silt and fine sand ranges depending on the standard; but for the clay threshold the widely adopted upper limit is 0.002 mm.

Step-by-Step Solution:

Verification / Alternative check:
Laboratory hydrometer analysis and pipette tests reference this 0.002 mm boundary for settling calculations based on Stokes’ law.

Why Other Options Are Wrong:
Values 0.04–1.0 mm lie in silt/sand ranges; they exceed the clay threshold.

Common Pitfalls:
Confusing mineralogy (e.g., “clay minerals”) with particle-size terms (“clay-size”); a soil can contain clay-size quartz that is not a clay mineral.

Final Answer:
0.002 mm