**Core Concept**
The human body maintains a tight regulation of hemoglobin levels in the bloodstream, which is essential for oxygen transport to various tissues and organs. Hemoglobin is a protein in red blood cells that binds oxygen and carbon dioxide, playing a crucial role in the respiratory and circulatory systems.

**Why the Correct Answer is Right**
There are approximately 5 liters of blood in an adult man, and each gram of hemoglobin binds to 1.34 milliliters of oxygen. The normal hemoglobin concentration in the blood is around 15 grams per deciliter (g/dL). Therefore, the total amount of hemoglobin in the circulating blood can be calculated as follows: 5 liters (blood volume) x 15 g/dL (hemoglobin concentration) x 100 mL/dL (concentration conversion) = approximately 7500 grams of hemoglobin.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because the amount of hemoglobin in the blood is not directly related to the total number of red blood cells.
* **Option B:** This option is incorrect because the amount of hemoglobin in the blood is not directly related to the total blood volume.
* **Option C:** This option is incorrect because the amount of hemoglobin in the blood is not directly related to the hemoglobin concentration in a single red blood cell.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the normal hemoglobin concentration in the blood is around 13.5-17.5 g/dL for adult men, and deviations from this range can indicate various conditions such as anemia, polycythemia, or hemolysis.

**Correct Answer:** C. 7500 grams.