Which of the following is a molecular disease?

Answer: Option D

Sickel cell anaemia is a molecular disease.
Any disease caused by an abnormality in a single protein, usually an enzyme is known as molecular disease.
Sickle cell anaemia results in an abnormality in the oxygen-carrying protein haemoglobin found in red blood cells. This leads to a rigid, sickle-like shape under certain circumstances.
Sickle cell disease occurs when a person inherits two abnormal copies of the β-globin gene that makes haemoglobin, one from each parent. This gene occurs in chromosome 11. Several subtypes exist, depending on the exact mutation in each haemoglobin gene.