**Core Concept**
Hemoglobin's affinity for oxygen is a crucial aspect of oxygen transport in the blood. The affinity for oxygen is determined by the binding of oxygen to the heme group of hemoglobin, which is influenced by various factors, including pH, temperature, and the presence of 2,3-bisphosphoglycerate (2,3 DPG).

**Why the Correct Answer is Right**
The correct answer is D. A decrease in 2,3 DPG. 2,3 DPG is an organic phosphate that binds to deoxyhemoglobin, causing a conformational change that reduces the affinity of hemoglobin for oxygen. When 2,3 DPG levels decrease, the affinity of hemoglobin for oxygen increases, allowing for more efficient oxygen delivery to tissues. This is particularly important in conditions such as anemia, where increased 2,3 DPG levels can further decrease oxygen delivery.

**Why Each Wrong Option is Incorrect**
**Option A:** An increase in CO2 actually decreases hemoglobin's affinity for oxygen, not increases it. CO2 binds to hemoglobin, causing a conformational change that reduces the affinity for oxygen.

**Option B:** A raise in temperature actually decreases hemoglobin's affinity for oxygen, not increases it. At higher temperatures, the hemoglobin molecule is more flexible, making it easier for oxygen to dissociate.

**Option C:** This option is unclear and does not make sense in the context of the question.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that 2,3 DPG levels are elevated in conditions such as anemia, chronic hypoxia, and certain types of kidney disease, which can lead to decreased oxygen delivery to tissues.

**✓ Correct Answer: D. A decrease in 2,3 DPG**