Occasionally, some lethal allele (like sickle cell) is unusually common (more than 50% heterozygotes). what does this tell us?

It tells us that there is selection pressure in that particular habitat that selects for individuals with these alleles.

In the case of sickle cell, malaria is the selection pressure as individuals with the sickle cell trait are more resistant to the deadly malaria.

That would be a working theory.

The answer to this is b