The theory of natural selection, proposed by Charles Darwin, states that individuals with favorable traits are more likely to survive, reproduce, and pass those traits on to their offspring. Conversely, individuals with unfavorable traits are more likely to struggle, die off, or fail to reproduce, thereby eliminating those traits from the population.

In other words, natural selection acts on the variation within a population, favoring individuals with traits that enhance their survival and reproductive success, while eliminating those with traits that hinder their ability to survive and reproduce.

So, option C accurately reflects this concept.

A. "There is no struggle for existence" is the opposite of what natural selection proposes.

B. "New species get better adaptation" is a consequence of natural selection, but it doesn't explain the mechanism.

D. "Unused structures disappear later" refers to the concept of vestigial structures, which is related to evolution, but not a direct explanation of natural selection.