Question

Which of the following is an example of reproductive isolation?
two populations of finches that cannot produce viable offspring
two populations of finches that have different feather colors
two populations of salamanders that live in different lakes
two populations of salamanders that have different tail lengths

Answer 1

"Two populations of finches that cannot produce viable offspring."

This directly describes reproductive isolation (a postzygotic barrier). The others describe differences (feather color, tail length) or geographic separation (different lakes) that don't necessarily prevent interbreeding.