Which of the following explains darvin's theory of natural selection

You didn't list the options. If you can comment the options perhaps I can be of use.

Are these the options?

A. More recent species of organisms are changed descendants of earlier species.

B. Individuals in a population with traits best suited for that environment survive and increase in number.

C. Present organisms are related to and descended from past organisms.

The answer would be B. Individuals in a population with traits best suited for that environment survive and increase in number. This answer best supports Darwin's theory because it is simply "Survival of the Fittest".

one would be wind another would be water density

if the type of the food available changes, then the frequency of beak types will change, because birds with beaks more suited to the available food will be more successful over time.