Question

which is left unexplained by the biological species concept, thereby weakening the concept?

hybridization in captivity

hybridization in natural settings

the existence of viable, but sterile hybrids (such as the mule)

Explanation:

The biological species concept defines a species as a group of organisms that can interbreed and produce fertile offspring in nature. Hybridization in natural settings challenges this concept as it shows that different species can interbreed in the wild, which the concept doesn't fully account for. Hybridization in captivity is less of a natural - occurring challenge, and the existence of viable but sterile hybrids like mules is somewhat accounted for as they don't produce fertile offspring.

Answer:

B. Hybridization in natural settings