**Core Concept:** Hemoglobin (Hb) is a protein found in red blood cells (RBCs) that transports oxygen from the lungs to the body's tissues and returns carbon dioxide back to the lungs. It is composed of two alpha (α) chains, two beta (β) chains, one gamma (γ) chain, and one delta (δ) chain.

**Why the Correct Answer is Right:** Hemoglobin consists of four chains: two alpha (α) chains, two beta (β) chains, one gamma (γ) chain, and one delta (δ) chain. These chains combine to form the functional structure of Hb, which enables it to bind and transport oxygen efficiently.

**Why Each Wrong Option is Incorrect:**

A. This option refers to the wrong type of chain (delta chain) and is incorrect because the delta chain is not involved in the functional structure of Hb.
B. This option refers to the wrong type of chain (gamma chain) and is incorrect for the same reason as option A.
C. This option refers to the wrong type of chain (alpha chain) and is incorrect because only two alpha chains are present in functional Hb, not three.
D. This option refers to the wrong type of chain (beta chain) and is incorrect because only two beta chains are present in functional Hb, not three.

**Clinical Pearl:** Understanding the correct structure of hemoglobin is crucial for understanding its function and various disorders related to it, such as sickle cell anemia and thalassemia, which occur due to mutations in the genes encoding for alpha, beta, gamma, or delta chains, respectively.

**Correct Answer:** D. beta chain

**Explanation:** The correct answer is option D, which refers to the beta chain. The beta chain is essential for the functional structure of hemoglobin, as it is one of the two beta chains present in functional hemoglobin. The other two chains are alpha chains (two of them). The other options (A, B, and C) refer to incorrect chains or the wrong number of chains, making them incorrect answers. Understanding the correct structure of hemoglobin is vital for grasping its function and various hemoglobinopathies, such as sickle cell anemia and thalassemia, which can result from mutations in genes encoding for alpha, beta, gamma, or delta chains, respectively.