Problem:

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, 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.

Required:

Question: What keeps the two populations separate?

a.) lack of hybrid viability

b.) temporal reproductive isolation

c.) habitat isolation

d.) behavioral reproductive isolation

Please provide correct answer.