Answer:

Introduction:
Hemoglobin is a protein found in red blood cells that is responsible for transporting oxygen from the lungs to the rest of the body. In humans, adult hemoglobin is composed of multiple subunits that come together to form a functional protein.

Number of Subunits:
The correct answer to the question is option A, which states that there are 4 subunits in human adult hemoglobin.

Explanation:
1. Hemoglobin consists of two pairs of subunits, known as globin chains. These chains are called alpha (α) and beta (β) chains.
2. Each globin chain is associated with a heme group, which contains an iron atom. This iron atom is responsible for binding to oxygen molecules.
3. In adult humans, there are two alpha chains and two beta chains present in each hemoglobin molecule. This arrangement is denoted as α2β2.
4. The alpha chains are encoded by two genes called HBA1 and HBA2, while the beta chains are encoded by a single gene called HBB. These genes are located on different chromosomes.
5. The alpha and beta chains are transcribed and translated separately, and they come together to form the complete hemoglobin protein during red blood cell maturation.
6. The interaction between the alpha and beta chains is crucial for the stability and function of the hemoglobin molecule.
7. The four subunits of hemoglobin form a complex quaternary structure that allows for efficient oxygen binding and release.

Summary:
In summary, human adult hemoglobin is composed of four subunits, with two alpha chains and two beta chains. These subunits come together to form a functional protein that is responsible for oxygen transport in the body.