Final answer:
The difference in the frequency of the sickle cell gene (S) between West Africa and the United States is best explained by selection for heterozygotes, as this provides resistance to malaria in regions where it is prevalent.
Step-by-step explanation:
The prevalence of the sickle cell gene (S) is higher in West Africa at a frequency of 0.15, compared to a frequency of 0.04 in the United States. The most likely explanation for this difference is selection for heterozygotes (C). In regions where malaria is common, such as West Africa, being heterozygous for the sickle cell trait (carrying one normal allele and one sickle allele) offers a survival advantage, as it provides resistance to malaria. This results in a higher frequency of the S allele in the gene pool due to natural selection. Conversely, in the United States, where malaria is not prevalent, the S allele does not confer a survival advantage, and thus its frequency is lower.