Final answer:
The term that describes why separated squirrel populations can't interbreed is speciation, which involves the evolution of new, genetically distinct species due to isolation.
Step-by-step explanation:
The term that explains why the separation of squirrels resulted in their inability to interbreed is d. speciation. Speciation occurs when populations of a single species become separated and evolve independently, often due to physical barriers or other factors that prevent them from interbreeding. This evolutionary process can lead to the appearance of new species with distinct characteristics and genetic divergences, rendering them incapable of successful mating with the original population.
Interspecific competition can sometimes influence this process by encouraging niche differentiation or the extinction of one species, but it is not the direct cause of the inability of the separated squirrel populations to interbreed. On the other hand, speciation is about the formation of new species through genetic isolation and evolutionary mechanisms. Thus, speciation is the correct term to describe the separation and subsequent reproductive isolation of squirrel populations.