The correct option is D Sibling species
Two or more species that are nearly identical in their appearance, yet reproductively isolated, are referred to as sibling or cryptic species. Sibling species are any of two or more related species that are morphologically nearly identical but are incapable of producing fertile hybrids.
Sibling species can only be identified by genetic, biochemical, behavioural, or ecological factors, and are thought to have become divergent very recently.
A deme is a term for a local population of organisms of one species, that actively interbreed with one another and share a distinct gene pool.
Clones are organisms that are exact genetic copies.
Cline is the gradual change in certain characteristics exhibited by members of a series of adjacent populations of organisms of the same species.