Answer:
b. disruptive
Explanation:
During extreme changes in the environmental conditions, natural selection favors the two or more extreme phenotypes due to their adaptive nature. During this, the organisms with intermediate phenotype are not favored. This type of natural selection is called disruptive selection. In the given example, Cameroons having large and small bills are favored which in turn are the two extreme phenotypes. Therefore, it represents an example of disruptive selection.