Why are finches important to darwin's idea?

It helps to give solid evidence of natural selection. The idea is that there are a number of finches in the Galapagos Islands. These birds vary slightly in appearance, in the function of their beaks and in size. Two species can eventually alter themselves because of the environmental conditions they live in. Some traits will be passed on or developed in one group and not in the other or vica versa. These finches are remarkable in that it did not take long for the changes to take place. It is probably one of the strongest reasons for believing in natural selection, because only those birds who could adapt to change could easily pass on these advantages to their offspring. The relatively short period of time to enact these changes is what makes the this example  so attractive.
 

Ithink that b is correct but just in case wait to see if someone else answers