**Core Concept**
Hemoglobin is a protein in red blood cells that carries oxygen to various tissues in the body. In adults, the most common form of hemoglobin is hemoglobin A, which is a tetramer composed of two alpha-globin and two beta-globin chains.

**Why the Correct Answer is Right**
Hemoglobin A is the most abundant form of hemoglobin in normal adult blood, accounting for approximately 95-98% of the total hemoglobin. It is produced by the bone marrow and is essential for transporting oxygen from the lungs to the tissues. The presence of hemoglobin A is a normal finding in healthy adults, and its levels can be affected by various factors such as altitude, smoking, and certain medical conditions.

**Why Each Wrong Option is Incorrect**
**Option A:** Hemoglobin F (Fetal Hemoglobin) is the most common form of hemoglobin in fetal blood, not in adult blood.
**Option B:** Hemoglobin A2 is a minor component of adult hemoglobin, making up approximately 2-3% of the total hemoglobin.
**Option C:** Hemoglobin H is a tetramer composed of four beta-globin chains and is associated with alpha-thalassemia, a genetic disorder.

**Clinical Pearl / High-Yield Fact**
The normal range for hemoglobin A in adults is approximately 13.5-17.5 g/dL for men and 12-16 g/dL for women. A hemoglobin level below 13.5 g/dL in men or 12 g/dL in women may indicate anemia.

**Correct Answer: A. Hemoglobin A**