**Core Concept**
The question is testing knowledge of a specific genetic mutation associated with a hemoglobinopathy. Beta globin is one of the two polypeptide chains that make up hemoglobin, the oxygen-carrying protein in red blood cells. Mutations in the beta globin gene can lead to various disorders, including sickle cell disease.

**Why the Correct Answer is Right**
The correct answer is a missense mutation at codon 6 of the beta globin gene, resulting in a substitution of glutamic acid (Glu) with valine (Val). This mutation leads to the production of abnormal hemoglobin S (HbS), which has a tendency to polymerize under low oxygen conditions, causing red blood cells to take on a sickle shape. This is a classic example of a pathogenic mutation in a beta globin gene, leading to sickle cell disease.

**Why Each Wrong Option is Incorrect**
**Option A:** A missense mutation at codon 42 of the beta globin gene leads to a substitution of glutamic acid (Glu) with lysine (Lys), resulting in hemoglobin C (HbC). This is a different type of hemoglobinopathy.
**Option B:** A nonsense mutation at codon 39 of the beta globin gene leads to a premature stop codon, resulting in beta zero thalassemia. This is a different type of hemoglobinopathy characterized by reduced beta globin production.
**Option C:** A frameshift mutation at codon 31 of the beta globin gene leads to a premature stop codon, resulting in beta plus thalassemia. This is a different type of hemoglobinopathy characterized by reduced beta globin production.

**Clinical Pearl / High-Yield Fact**
The sickle cell mutation is the most common genetic disorder worldwide, affecting millions of people, particularly in sub-Saharan Africa and the Mediterranean region. It is inherited in an autosomal recessive pattern, meaning an individual must inherit two copies of the mutated gene (one from each parent) to express the disease.

**Correct Answer: D. Missense mutation at codon 6 of the beta globin gene, resulting in a substitution of glutamic acid (Glu) with valine (Val).**