Explanation:

Classification of organisms is done on the basis of their characteristics and features. Organisms that share common characteristics are grouped together into categories. These categories are arranged in a hierarchical manner, with the most closely related organisms being placed in the same category.

The category that groups together a set of closely related classes is called a Phylum. A Phylum is a taxonomic rank that comes below the Kingdom and above the Class. It is a major category in the classification of living organisms.

The classification of living organisms is based on the following categories:

1. Kingdom: The highest level of classification, which divides organisms into five major groups - Monera, Protista, Fungi, Plantae, and Animalia.

2. Phylum: The second level of classification, which groups organisms based on their body plan, structure, and development.

3. Class: The third level of classification, which groups organisms based on their characteristics such as the number of limbs, presence or absence of a backbone, etc.

4. Order: The fourth level of classification, which groups organisms based on their evolutionary relationships.

5. Family: The fifth level of classification, which groups organisms based on their genetic relationships.

6. Genus: The sixth level of classification, which groups organisms based on their physical and genetic similarities.

Conclusion:

The correct answer to the question is b. A group of closely related classes is called a Phylum. It is a major category in the classification of living organisms and comes below the Kingdom and above the Class.