The level of classification that contains the most organisms is the domain.
Classification in biology is a way to group and categorize living organisms based on their shared characteristics. The taxonomic hierarchy consists of several levels, including domain, kingdom, phylum, class, order, family, genus, and species.
The domain is the highest rank in this hierarchy, and it encompasses the broadest range of life forms. There are three recognized domains: Archaea, Bacteria, and Eukarya. Each of these domains contains a vast number of organisms, ranging from single-celled bacteria to complex multicellular organisms like plants, animals, and fungi.
In contrast, lower levels of classification, such as species or genus, contain fewer organisms because they represent more specific groupings based on closer evolutionary relationships. Therefore, if you are looking for the classification level that encompasses the most diversity and number of organisms, the domain is the correct answer.