You have several hives of honeybees, Apis mellifera, that you keep in your backyard. Some bees uncap and remove dead larvae from cells, while others leave dead larvae in the cells; this difference is genetically determined. After a particularly hot summer in which many bees die from disease, you notice that the only hives remaining are the ones in which bees uncap and remove dead larvae. You have witnessed
A. a population-level change in the frequency of a behavioral trait
B. evolution by natural selection
C. a new species forming
D. A and B

Some Apis mellifera bees genetically inherit the hygienic behaviour to remove the dead and diseased larvae from beehives by uncapping the sealed cells. This activity maintains the overall health of the bee colony by protecting it from unwanted pests and pathogenic diseases. Usually both hygienic and non hygienic bees survive well.

Here, in hot summer many bees had perished due to disease and hot climate. Accumulation of dead larvae in hives of non hygienic bees would have invited more pathogens and created infectious conditions in summer. Hence their hives could not survive. Now the remaining hygienic bees will mate with each other and the offspring will have more chances to be born with the hygienic trait. Hence the frequency of alleles determining this trait will increase in this population. This would also be an example of natural selection as hygienic bees were preferred by the nature over non hygienic ones to survive in hot season.