Which taxonomic rank is the most specific, capable of producing fertile offspring?

The most specific taxonomic rank that is capable of producing fertile offspring is species. In biological classification, a species is defined as a group of organisms that can interbreed and produce fertile offspring under natural conditions. This characteristic is essential for maintaining the gene pool and ensuring the continuation of the species.

Other taxonomic ranks such as genus, family, and order represent broader categories that encompass multiple species. For example, a genus groups together species that are closely related and share a common ancestor, but it may include several different species, some of which may not be capable of interbreeding. Similarly, family encompasses multiple genera, and order includes multiple families, further broadening the groupings and reducing specificity.

Thus, while the other ranks are important for classification and understanding evolutionary relationships, species is the most specific level where the ability to produce fertile offspring is a defining characteristic.