**Core Concept**
Hemoglobinopathies and disorders of erythropoiesis can lead to alterations in the normal structure and function of hemoglobin. However, in this question, we are looking for conditions where the structure of hemoglobin is normal, but there is a reduction in blood volume.

**Why the Correct Answer is Right**
The correct answer involves conditions that result in a reduction in blood volume, also known as hypovolemia. This can occur due to various reasons such as dehydration, hemorrhage, or excessive diuresis. In these conditions, the structure of hemoglobin remains normal, but the reduced blood volume can lead to decreased oxygen delivery to tissues. This is particularly relevant in conditions like hemorrhagic shock, where there is significant blood loss leading to a decrease in blood volume.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is likely referring to conditions like thalassemia, where there is a defect in hemoglobin synthesis. This would result in abnormal hemoglobin structures, not normal structures.

**Option B:** This option is likely referring to conditions like sickle cell disease, where there is a mutation in the hemoglobin gene leading to abnormal hemoglobin structures. This would not be a correct answer.

**Option C:** This option is likely referring to conditions like polycythemia vera, where there is an increase in blood volume due to overproduction of red blood cells, not a decrease.

**Clinical Pearl / High-Yield Fact**
Remember that hypovolemia can occur due to various reasons, including hemorrhage, dehydration, and excessive diuresis. It's essential to identify the underlying cause of hypovolemia to provide appropriate treatment.

**Correct Answer:** C.