According to scientific thought, when does one species of organisms become two species?

It happens when the two groups become sufficiently different and the two can no longer interbreed. A known example of speciation is the London underground mosquito where it formed a new species within the existing genus. There are examples in micro-evolution but not in macro-evolution.

The answer for your MCQ would be:

C. when one population cannot interbreed with the other population

This process is called speciation and a fertile offspring being produced is a requirement, if the offspring produced are infertile such as that of a cross between a lion and a tiger they are considered distinctly different species.