Sickle cell anaemia is

The correct option is B caused by a change in a single base pair of DNA
Sickle cell anaemia is caused due to the substitution of glutamic acid by valine at the $6^{\mathrm{th}}$ position of the beta globin chain of haemoglobin. This occurs due to change at a single base pair in the gene (a segment of DNA encoding for a specific protein) encoding the beta globin chain. The substitution has been shown in the image below.

This results in the production of defective haemoglobin molecules which results in the distortion in the shape of RBCs. The biconcave RBCs turn sickle-shaped and elongated but they still lack nucleus.
It is an autosomal linked recessive trait. ${\mathrm{Hb}}^A$ represents the normal allele while ${\mathrm{Hb}}^S$ represents the mutated allele.