What is the lowest taxonomic level of biological classification that includes organisms capable of reproduction resulting in fertile offspring?

The lowest taxonomic level of biological classification that includes organisms capable of reproduction resulting in fertile offspring is species. This is because the concept of a species is commonly defined as a group of organisms that can interbreed and produce viable, fertile offspring under natural conditions. This reproductive isolation is a fundamental criterion for distinguishing one species from another.

In contrast, while genera, families, and orders also group organisms based on shared characteristics, they encompass broader categories that may include multiple species. For example, a genus contains several species that are closely related, while a family includes multiple genera. Therefore, species is the most specific category that directly pertains to the ability of organisms to reproduce and produce fertile offspring.