Natural selection causes changes in populations and phenotypes.
The correct answer is "populations." Natural selection is a process in which individuals with advantageous traits are more likely to survive, reproduce, and pass on their genes to the next generation. Over time, this leads to changes in the genetic composition of a population, as individuals with less advantageous traits are less likely to survive and reproduce. So, natural selection acts on populations, shaping their genetic makeup and increasing the frequency of traits that enhance survival and reproduction.
