Explanation:
Base exchange capacity (CEC) refers to the ability of a clay mineral to hold and exchange cations with the surrounding environment. It is a measure of the clay's ability to retain nutrients and other ions.

Among the given options, kaolinite has the least base exchange capacity. Here's why:

1. Structure:
Kaolinite is a 1:1 clay mineral, meaning it consists of one layer of tetrahedral silica and one layer of octahedral alumina. The layers are held together by weak van der Waals forces, resulting in a relatively low surface area and CEC.

2. Lack of isomorphous substitution:
Kaolinite does not undergo significant isomorphous substitution, which is the replacement of one element by another of similar size and charge within the crystal structure. Isomorphous substitution increases the CEC by providing more sites for ion exchange. In kaolinite, the structure is relatively stable, and there are limited opportunities for cation exchange.

3. Alumina content:
The CEC of clay minerals is primarily influenced by the amount of alumina in their structure. Alumina has a high affinity for cations and can exchange them readily. Kaolinite has a lower alumina content compared to other clay minerals like montmorillonite, illite, and halloysite, resulting in a lower CEC.

4. Lack of expandable layers:
Kaolinite does not have expandable layers like montmorillonite, which can accommodate a high number of exchanged cations. The absence of expandable layers further limits the CEC of kaolinite.

In summary, kaolinite has the least base exchange capacity among the given options due to its 1:1 structure, lack of isomorphous substitution, lower alumina content, and the absence of expandable layers.