Natural selection causes ...

a: plants and animals to produce more offspring than can survive

b:changes in the environment

c:only favorable traits to exist in a population

d:changes in the frequencies of certain alleles in a population

A is your answer to your question: plants and animals to produce more offspring than can survive

C is correct. The environment selects for or against specific alleles (traits). If the trait is selected for, it increases an organisms chance to survive and reproduce. This will increase (change) the frequency of the characteristic (allele) in the population.