**Core Concept:** Haemoglobin is a tetrameric protein composed of four polypeptide chains (two α-globin and two β-globin). It plays a crucial role in transporting oxygen from the lungs to the body's tissues and back, and is responsible for the red color of blood. 2,3-Diphosphoglycerate (DPG) is an anion that modulates the affinity of haemoglobin for oxygen. The pH of blood affects the ionization state of DPG, which in turn affects the oxygen-binding properties of haemoglobin.

**Why the Correct Answer is Right:** The correct answer is D. Hemoglobin's R-state (low oxygen affinity) is favored at higher pH values (e.g., in the tissues), while the T-state (high oxygen affinity) is favored at lower pH values (e.g., in the lungs). 2,3-Diphosphoglycerate (DPG) is an anion that modulates haemoglobin's affinity for oxygen by shifting the equilibrium between the R and T-state.

**Why Each Wrong Option is Incorrect:**

A. β-globin chains: While β-globin is a part of the haemoglobin molecule, the correct moiety binding DPG is not specified.
B. α-globin chains: α-globin chains are not involved in binding DPG, as the correct moiety is not specified.
C. All four globin chains: The question specifies binding to a specific moiety, and the correct answer is D.

**Clinical Pearl:** Understanding the pH-dependent oxygen affinity of haemoglobin is crucial for understanding the pathophysiology of anemias and the therapeutic approach in managing them. For example, in sickle cell anemia, the abnormal HbS (sickle cell hemoglobin) has a higher affinity for oxygen due to the presence of a single amino acid substitution, leading to vaso-occlusive crises in patients. In contrast, thalassemias (α- or β-globin chain deficiencies) result in lower oxygen affinity, causing tissue hypoxia and complications.

**Correct Answer: D. β-globin chains**

The correct answer is D. β-globin chains because they are the ones responsible for the binding of DPG (2,3-Diphosphoglycerate), which is an anion that modulates the oxygen affinity of hemoglobin. The pH-dependent binding of DPG to haemoglobin results in a decrease in oxygen affinity, allowing for efficient release of oxygen in tissues. This is important for understanding the pathophysiology of anemias and the therapeutic approach in managing them.