Sickle cell anemia is caused by

The correct option is B Point mutation of the haemoglobin gene
Hemoglobin has four protein subunits, two $\alpha$-globin and two $\beta$-globin. The Hb${}^b$ gene codes the $\beta$ chain. A missense mutation, replacement of A by T at the 17th nucleotide of the Hb${}^b$ gene changes the codon GAG (glutamic acid) to GTG (which encodes valine). Thus, the disease is caused by a point mutation (change in single nucleotide base) in the gene; option B is correct.

The mutations that introduce a stop codon in between the reading frame causing premature termination of protein synthesis results in truncated protein; options A and D are incorrect as there is the substitution of amino acids, no premature termination or complete suppression of haemoglobin synthesis. The mutated allele Hb${}^s$ encodes the abnormal haemoglobin molecules which stick to one another and cause stiffness and sickle shape of red blood cells. The sickle-shaped RBCs block and damage the vital organs and tissue; they do not show an altered affinity for oxygen, option C is incorrect.