The correct answer is answer C.
This is called sickle cell anemia. The mutation here is a valine in place of glutamic acid at position 6 of the B-globin chain. The resulting mutated hemoglobin is called HbS. When deoxygenated the HbS changes conformation such that it polymerizes and crosslinks, misshaping the cell.
The hemoglobin can still function. It can still bind and unbind oxygen, and therefore deliver oxygen. In fact when HbS is oxygenated the RBC will return to its normal shape it is just the deoxygenated form of HbS that causes the sickling.
