Answer:
Option D, spatial reproductive isolation—the only reason these two species don’t mate is because of the inability of the middle land to be occupied
Step-by-step explanation:
It is clear from the question that the two bird populations with different color are able to mate and reproduce viable and fertile offspring when brought together. Hence, the only problem due to which these two bird population were not breeding with each other was their distant location and habitat.
Location and habitat are characteristic of spatial distribution and hence they were separate because of spatial reproductive isolation
Hence, option D is correct