Final answer:
Favorable traits build in a population over time as a result of natural selection, which favors alleles that confer advantageous phenotypes for survival and reproduction, ultimately increasing their frequency within the gene pool.
Step-by-step explanation:
The statement that describes how favorable traits in a population relate to natural selection is that they build in the population over time. Natural selection acts on heritable traits and selects for beneficial alleles that allow for environmental adaptation, increasing their frequency in the population while selecting against deleterious alleles, which decreases their frequency.
An allele that confers a phenotype improving survival or reproductive success will be passed on to offspring, who will also likely be more successful, leading to an increased frequency of that allele in the population.