Na-montmorillonite is the most active clay material among the options given. Let's understand why this is the correct answer.

1. The Structure of Montmorillonite:
Na-montmorillonite is a type of smectite clay mineral. It belongs to the group of 2:1 layer silicates, which means it consists of two tetrahedral sheets sandwiching a central octahedral sheet. The layers are held together by weak van der Waals forces, allowing them to expand and contract easily. This structure gives montmorillonite its unique properties.

2. Cation Exchange Capacity (CEC):
One of the key factors that determine the activity of a clay material is its Cation Exchange Capacity (CEC). CEC refers to the ability of clay minerals to attract and exchange cations (positively charged ions) in solution. Higher CEC values indicate higher reactivity and greater ability to adsorb and release cations.

3. Sodium-Ion Dominance:
Na-montmorillonite is characterized by a high CEC due to the presence of sodium ions (Na+) within its structure. These ions are loosely attached and can be easily exchanged with other cations in solution. The high CEC of Na-montmorillonite allows it to effectively adsorb and retain various cations, such as calcium, magnesium, and potassium.

4. Swelling and Shrinkage:
Another important characteristic of Na-montmorillonite is its ability to swell and shrink in response to changes in water content. This property is known as the clay's swelling behavior. When the clay is hydrated, water molecules enter the interlayer space between the layers, causing them to expand. Conversely, when the clay dries, the water is released, and the layers contract.

5. Reactivity and Plasticity:
The combination of high CEC and swelling behavior gives Na-montmorillonite enhanced reactivity and plasticity. Reactivity refers to the clay's ability to chemically interact with other substances, while plasticity refers to its ability to be molded or shaped without breaking. These properties make Na-montmorillonite highly suitable for applications such as soil stabilization, drilling muds, and as a component in geosynthetic clay liners.

In summary, Na-montmorillonite is the most active clay material because of its high CEC, sodium-ion dominance, swelling behavior, and reactivity. These properties make it a versatile and valuable clay mineral in various engineering and industrial applications.