What classification level includes organisms that can interbreed and produce fertile offspring?

The classification level that includes organisms capable of interbreeding and producing fertile offspring is species. The concept of species is fundamental in biology because it reflects a group of organisms that share common characteristics and genetic similarities, as well as the ability to reproduce within the group.

When two organisms are identified as belonging to the same species, they can mate and produce offspring that are also viable and can reproduce. This ability to interbreed is a key defining factor of the species concept, emphasizing both reproductive compatibility and genetic continuity among individuals of that group.

The other classification levels, such as class, order, and family, represent broader categories that encompass multiple species, but they do not define the specific reproductive capabilities and relationships that are inherent to the concept of a species. This makes species the most appropriate answer to the question posed.