Answer:
Option A, The two forms interbreed successfully where their habitats overlap
Step-by-step explanation:
The two species of warbler i.e myrtle warbler and Audubon's warbler are capable enough to interbreed when their habitat overlap by maintaining postzygotic selection against hybrids.
These two warblers do not interbreed freely but they do so on routine basis in a narrow zone at the northern edge of the Audubon form’s range in western Canada to hybridize
Hence option A is correct