Final answer:
Individuals whose red blood cells are the correct shape and can resist malaria have a better chance of reaching reproductive age. This is known as heterozygote advantage.
Step-by-step explanation:
The correct answer is A. heterozygote advantage. In the case of malaria, individuals who are heterozygous (AS) with the sickle-cell allele are resistant to malaria. This means they have a better chance of surviving and reaching reproductive age compared to individuals who are homozygous for the normal allele (AA) or for the sickle-cell allele (SS). This advantage helps maintain the sickle-cell allele in the population.