Step 1: Understanding the Concept:
Atterberg limits define the critical water content levels at which fine-grained soils transition between different states of mechanical behavior.
Step 3: Detailed Explanation:
The physical state of a cohesive soil depends heavily on its water content (moisture content).
As moisture increases, the soil state transitions through several stages:
\[ \text{Solid} \xrightarrow{\text{Shrinkage Limit}} \text{Semi-Solid} \xrightarrow{\text{Plastic Limit}} \text{Plastic} \xrightarrow{\text{Liquid Limit}} \text{Liquid} \]
These boundaries (Liquid Limit, Plastic Limit, and Shrinkage Limit) are specific measures of soil moisture content.
Hence, the Atterberg limits represent the critical moisture stages of fine-grained soil behavior.
Step 4: Final Answer:
The correct option is 1, which corresponds to Soil moisture.