Human populations living in areas of endemic malaria have a selective advantage if they carry:

Human populations living in areas of endemic malaria have a selective advantage if they carry the allele for sickle cell anemia.
Sickle-cell anemia is a blood disorder which occurs when a person inherits two abnormal alleles of the haemoglobin gene. In this disease red blood cells (erythrocytes) are sickle-shaped and as a result, they poorly transport oxygen to cells throughout the body.

Ithink that b is correct but just in case wait to see if someone else answers