Sickle-cell disease is clearly a debilitating disease, but why does the mutated beta-globin allele persist? a. the mutated allele offers some protection (selective advantage) against malaria when in the heterozygous state with the normal allele. b. mutations keep the mutated allele at high frequency throughout the world. c. genetic drifts prevents the mutated allele from disappearing. d. the mutated allele once provided a selective advantage in homozygous individuals, and may once again do so in the future. e. none of the other answer choices are correct.

The answer is A.

The parasite causing malaria will try to reproduce inside normal red blood cells.
In case of having sickle-cell disease, the erythrocyte doesn't have a normal shape (it's more curved) and our immune system thinks that this cell should be eliminated due to the deformity.
Since the erytrocyte is going to be eliminated, the parasites inside will also be. This is a survival advantage, and so this allele tends to increase.
However, the disease only happens in heterozygotic people(with one abnormal allele while the other is normal) because in this cases the deformity is not so severe to cause anemia.

The zone your looking for is the intertidal zone.