**Core Concept**
The oxygen hemoglobin dissociation curve (ODC) is a graphical representation of the relationship between the partial pressure of oxygen and the saturation of hemoglobin with oxygen. The curve shifts in response to various physiological and pathological conditions, affecting the binding of oxygen to hemoglobin and the release of oxygen to tissues.

**Why the Correct Answer is Right**
A decrease in hydrogen ions (H+) concentration, also known as an increase in pH, is associated with a left shift of the ODC. This means that hemoglobin has a higher affinity for oxygen and is less likely to release oxygen to tissues. A left shift is typically seen in conditions such as alkalosis or increased 2,3-bisphosphoglycerate (2,3-BPG) levels. In contrast, a decrease in H+ concentration would not cause a right shift of the ODC.

**Why Each Wrong Option is Incorrect**
**Option A:** Hyperthermia is associated with a right shift of the ODC, as increased body temperature decreases the affinity of hemoglobin for oxygen, allowing for more efficient oxygen delivery to tissues.
**Option B:** Decreased pH, or acidosis, is also associated with a right shift of the ODC, as the decrease in H+ concentration increases the affinity of hemoglobin for oxygen.
**Option D:** Increased CO2 levels lead to a right shift of the ODC, as CO2 binds to hemoglobin and decreases its affinity for oxygen, facilitating oxygen delivery to tissues.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the Bohr effect describes the decrease in oxygen affinity of hemoglobin in response to increased CO2 and decreased pH, leading to a right shift of the ODC.

**✓ Correct Answer:** C. Decreased H+