Answer:
a. The two forms interbreed often in nature, and their offspring survive and reproduce well.
Explanation:
A species is a group of organisms that can breed with each other and reproduce viable offsprings that are also capable of reproducing in future.
Here, Audubon's warbler and Myrtle warbler had separate habits which might or might not be similar. Their food requirements also might or might not be similar. But they were found to interbreed and give rise to offsprings who survived and reproduced well so they belonged to the same species of yellow rumped warbler.