**Core Concept**
Beta thalassemia is a genetic disorder characterized by mutations in the HBB gene encoding the beta-globin subunit of hemoglobin. This leads to reduced or absent beta-globin production, causing anemia and other clinical manifestations.

**Why the Correct Answer is Right**
The most common mutation in beta thalassemia is a point mutation in the HBB gene, resulting in a substitution of glutamic acid with valine at position 6 (Glu6Val). This mutation leads to the production of a non-functional beta-globin protein, which in turn causes a significant reduction in the production of the beta-globin subunit. The resulting hemoglobin, known as HbS, is unable to bind oxygen effectively, leading to anemia and other complications.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not specify the correct gene mutation associated with beta thalassemia.
**Option B:** This option is incorrect because it is a common mutation in alpha thalassemia, not beta thalassemia.
**Option C:** This option is incorrect because it is a mutation in the HBA2 gene, which is associated with alpha thalassemia.

**Clinical Pearl / High-Yield Fact**
The beta thalassemia mutation is usually inherited in an autosomal recessive pattern, meaning that an individual must inherit two copies of the mutated gene (one from each parent) to express the condition.

**Correct Answer: C. A mutation in the HBB gene, specifically the substitution of glutamic acid with valine at position 6 (Glu6Val).**