The sixth mutant codon of beta globin gene causing polymerization of Haemoglobin and change in RBC shape is :

(A) GUG
Sickle cell anaemia is caused by the substitution of Glutamic acid (Glu) by Valine (Val) at the sixth position of the beta globin chain of the haemoglobin molecule.
This substitution of the amino acid occurs due to a single base substitution at the sixth codon of the beta globin gene from GAG to GUG.
The normal codon is GAG, which codes for Glutamic acid, while the mutant codon is GUG, which codes for Valine.
The presence of Valine leads to the polymerization of the mutant haemoglobin molecule under low oxygen tension, causing the change in the shape of the RBC from biconcave disc to an elongated sickle-like structure.