A) The two forms are observed to interbreed successfully where their habitats overlap.
Explanation:
Under biological classification several taxonomic ranks assigned. The animals have been grouped into these taxons on the basis of similarities and dissimilarities.
Degree of similarities increases as we move from kingdom to species. Species is the lowest taxonomic rank and all the organisms belonging to a species has highest degree of similarities.
They are capable of interbreeding among themselves.
Since,myrtle warbler and Audubon's warble were able to interbreed so they were placed under single species.