Sickle-cell anaemia is caused due to the replacement of:

The correct option is B Glutamic acid at the 6th position by valine
Sickle-cell anaemia is a monogenic disorder in which the RBCs becomes sickle-shaped. It is caused by an alteration in haemoglobin structure. The glutamic acid is replaced by valine at the sixth position. Due to which the structure of the haemoglobin molecule changes. This changes the RBCs from biconcave to sickle-shaped. The sickle shaped cells cannot pass through the blood vessels. This leads to reduced blood circulation and increased clotting. As a consequence of which, enough oxygen cannot reach the cells of the body.