Two Populations of Birds with Somewhat Different Coloration Live on Opposite

Two populations of birds with somewhat different coloration live on opposite sides of a peninsula. The habitat between the populations is not suitable for these birds. When birds from the two populations are brought together in captivity, they interbreed and produce fertile young whose appearance is intermediate between the two parents. In the wild, the two populations remain separate as a result of which of the following factors?

A) temporal reproductive isolation
B) lack of hybrid viability
C) behavior isolates reproductive activities
D) habitat isolation