Question

Natural selection in a population leads to evolution when:

Answer 1

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)