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

The answer is A. two populations of finches that cannot produce viable offspring

Option A

Explanation:

Reproductive isolation can be defined as the condition in which the  environment imposes external barriers on the organism.

These external barriers can be river, mountain, between two incipient species. In case the species are separated  and they are not able to produce viable offspring is an example of reproductive isolation.

Basically in case of the reproductive isolation two population are not able to produce viable offspring and they avoid mating with the members of the other group.

that is wrong, its not thymus.