The concept of similarity among characters is based on the taxonomic classification of living organisms. Taxonomy is the branch of biology that deals with the naming, identification, and classification of organisms based on their morphological, physiological, and genetic characteristics. The classification system is hierarchical and based on the principle of nested sets.

Hierarchy of Taxonomic Classification:

1. Domain
2. Kingdom
3. Phylum
4. Class
5. Order
6. Family
7. Genus
8. Species

Each level of classification represents a group of organisms with similar characteristics. The higher the level of classification, the more general the characteristics of the organisms within the group. The lower the level of classification, the more specific the characteristics of the organisms within the group.

Among the given options, class is the least similar in terms of character because the organisms within the same class share fewer similarities compared to those in the same family, genus, or species. Here are some reasons why class has the least similarity among characters:

1. Wide Range: Class is a broad taxonomic category, and organisms within the same class can have diverse characteristics. For example, the class Mammalia includes organisms with different modes of reproduction, feeding habits, and body structures, such as whales, bats, and humans.

2. Shared Characteristics: Organisms within the same class share some characteristics, but these characteristics are not unique to the class. For example, all mammals have hair and mammary glands, but other organisms such as birds and reptiles have feathers and scales, respectively.

3. Evolutionary Relationships: The classification of organisms into classes is based on their evolutionary relationships, which can be complex and sometimes controversial. Some organisms may share similarities with organisms outside their class due to convergent evolution or other factors.

In conclusion, the least similarity among characters is observed in the class because it is a broad taxonomic category that includes organisms with diverse characteristics and shared characteristics that are not unique to the class.