Darwin discovered twenty different species of finches in the Galapagos islands. He hypothesized that as new islands arose, finches colonized the islands and rapidly changed into new species, depending on what food was available on each new island. The rapid formation of many new species, as in this case, is called

Answer : The correct answer is -

Adaptive radiation.

Adaptive radiation is an evolutionary process that describes rapidformation of new species from a group of organisms that have common ancestral species.

It occurs particularly when group of organisms move to different environmental conditions ( with availability of new resources) that leads to new challenges for the organism, which adapt to it and eventually forms new traits, thereby forming new species.

As per the Darwin's discovery, finches colonized the islands and rapidly changed into new species, depending on what food was available on each new island. This leads to rapid formation of new species.

Thus, it is an example of adaptive radiation.