**Core Concept**
The concept being tested here is the volume of blood contained in one unit of blood, its relation to the hemoglobin (Hb) content, and how it affects the overall Hb concentration in the body. This involves understanding the hematocrit, which is the proportion of blood volume that is occupied by red blood cells.

**Why the Correct Answer is Right**
One unit of blood is equivalent to approximately 350-400 mL of blood. Since hemoglobin is present in red blood cells, the amount of hemoglobin in one unit of blood can be estimated. A typical adult has about 5 liters of blood with a total hemoglobin content of approximately 150 g. The hemoglobin concentration in blood is approximately 15 g/100 mL or 150 g/L. Therefore, one unit of blood (350-400 mL) would contain approximately 50-60 g of hemoglobin. This amount of hemoglobin would increase the hemoglobin concentration in the body by about 1 g/dL when distributed throughout the total blood volume.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because a 0.1 g/dL increase in hemoglobin concentration is too small to be associated with one unit of blood.

**Option B:** This option is incorrect because a 0.5 g/dL increase in hemoglobin concentration is still too small to be associated with one unit of blood.

**Option D:** This option is incorrect because a 2 g/dL increase in hemoglobin concentration is too large to be associated with one unit of blood.

**Clinical Pearl / High-Yield Fact**
In transfusion medicine, it's essential to remember that one unit of blood typically contains approximately 50-60 g of hemoglobin, which corresponds to a 1 g/dL increase in hemoglobin concentration when distributed throughout the total blood volume.

**✓ Correct Answer: C. 1gm%**