Concept:Natural selection operates on the differences that exist among individuals of a species.
Explanation:Natural selection is the process by which organisms with advantageous traits survive and reproduce more successfully.
For this process to occur, individuals within a population must differ from one another in their traits.
These differences are known as variation.
Without variation, there would be no differences for nature to select among.
Thus, variation in organisms is the essential basis for natural selection.
Answer:C. variation in organisms