Which answer best illustrates evolution by natural selection? a

 Best   the answer here is A. This is pretty much the definition for natural selection; a basic example is how Darwin theorised giraffes got their long necks. Over many generations, giraffes stretched to reach high branches = food. Only the taller giraffes reached the food, leading these to be the ones who would survive and hence breed, passing on their characteristic of a long neck. Fast forward many generations, and you have long necks in the entire species. 
Hope that helped a little :)

Cells of an organism tell each other when predators are near.