At what point are two populations descending from the same ancestral stock considered separate species?

Explanation

The correct answer is Option A: When they can no longer produce viable, fertile offspring. This aligns with the biological species concept, which defines species based on their ability to interbreed and produce fertile offspring. Option B is incorrect because morphological differences alone do not define species. Option C contradicts the definition of separate species. Option D is incorrect because geographical separation does not necessarily result in speciation. Option E is incorrect as both B and C are not valid reasons to consider populations as separate species.