**Core Concept**
The underlying principle being tested is the composition of adult **haemoglobin**, which is a crucial protein in red blood cells responsible for oxygen transport. Adult haemoglobin is composed of four polypeptide chains. The structure and function of haemoglobin are fundamental to understanding various physiological and pathological processes.

**Why the Correct Answer is Right**
The correct composition of adult haemoglobin is two **alpha (α) chains** and two **beta (β) chains**, denoted as haemoglobin A (HbA) or **α2β2**. This specific arrangement of chains is vital for the proper binding and release of oxygen. The alpha and beta chains are encoded by different genes and have distinct amino acid sequences, which contribute to the overall function of haemoglobin.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not represent the correct composition of adult haemoglobin.
**Option B:** Similarly, this option is incorrect as it misrepresents the tetrameric structure of adult haemoglobin.
**Option D:** This option is also incorrect because it does not accurately describe the composition of adult haemoglobin.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that abnormalities in the chains of haemoglobin can lead to various diseases, such as **sickle cell disease** (resulting from a mutation in the beta chain) and **thalassemias** (resulting from mutations affecting the production of alpha or beta chains).

**Correct Answer:** Correct Answer: C. α2β2.