**Core Concept**
Sickle cell anaemia is a genetic disorder caused by a mutation in the HBB gene that codes for the beta-globin subunit of haemoglobin. This mutation leads to the production of abnormal haemoglobin, known as sickle haemoglobin (HbS), which causes red blood cells to assume a sickle shape under certain conditions.

**Why the Correct Answer is Right**
The correct answer is a type of point mutation, specifically a single nucleotide polymorphism (SNP), where a single base change occurs in the HBB gene. This mutation is a result of a nucleotide substitution, where glutamic acid (Glu) is replaced by valine (Val) at position 6 of the beta-globin chain. This substitution leads to a change in the haemoglobin's hydrophobic properties, causing it to polymerize under deoxygenated conditions and form the characteristic sickle-shaped red blood cells.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because it does not specify the type of mutation seen in sickle cell anaemia.
* **Option B:** This option is incorrect because it refers to a type of mutation that is not typically associated with sickle cell anaemia.
* **Option C:** This option is incorrect because it does not accurately describe the type of mutation seen in sickle cell anaemia.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that sickle cell anaemia is an autosomal recessive disorder, meaning that an individual must inherit two copies of the mutated gene (one from each parent) to express the disease.

**Correct Answer: A. Point mutation**