Answer:
b. successfully interbreed
Explanation:
At first, these two kinds of warblers are described as two different species because of their ranges. However, if they are described now as a single species, that means that they can mate between them. A species, in general terms, is defined as a reproductive unit of organisms, so if a group of organism can produce fertile progeny, it is called "species".
Living in the same area, or having an identical appearance are not criteria to call a group of individuals "species". So, options a and c are wrong.
Option "d. are merging to form a single species" implies than before, the two types of warbles were different species, and now they formed a new single species. This is not what the rubric says. In this example, the two warbles always have been a single species, but before they were wrongly categorized, probably because it wasn't known that they were able to interbreed.
