Final answer:
The Grants showed that natural selection in Galapagos finches is evidenced by the size and shape of their beak, which influenced survival and reproduction in response to food resource availability.
Step-by-step explanation:
The characteristics of Galapagos finches that helped the Grants show the results of natural selection are C. The size and shape of their beak. The Grants' research found that during a drought, the finches with larger beaks could eat larger seeds, giving them a better chance of survival compared to those with smaller beaks. As a result, these birds were likelier to reproduce, leading to an increase in the average beak size in the finch population, providing concrete evidence of evolution by natural selection. Moreover, when the conditions changed and small seeds became more abundant, the finches with smaller beaks had a survival advantage, which was also reflected in the population's beak size.