Two Populations of Birds with Somewhat Different Colouration Live on Opposite

Multiple Choice

Two populations of birds with somewhat different colouration 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, they produce young whose appearance is intermediate between the two parents. These offspring will breed with each other or with birds from either parent population, and all offspring of these pairings appear intermediate to various degrees.
What keeps the two populations separate?

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