Natural selection in a population leads to evolution when: a. every new generation of the population is a new species. b. an offspring does not have all the same traits as its parents. c. an organism cannot reproduce because of a mutation. d. allele frequencies of some traits change in the population.

D) allele frequencies of some traits change in the population

Alleles can be phenotype (how they appear) and natural selection is when an organism changes in a way that helps them survive.

It can be the way their beaks are formed that gives them a better chance of catching food among other stuff (Darwin's investigation)
Or how they change their color to adapt to the sudden change in tree coloration (Pepper moths)

Sometimes forms of cancer can be the cause.