**Core Concept**
Oxygen delivery to tissues is determined by **cardiac output** and **arterial oxygen content**. A reduction in either of these components can significantly impact oxygen delivery.

**Why the Correct Answer is Right**
A 50% reduction in **hemoglobin** would significantly decrease the arterial oxygen content, as hemoglobin is the primary carrier of oxygen in the blood. This reduction would lead to a substantial decrease in oxygen delivery to tissues.

**Why Each Wrong Option is Incorrect**
**Option A:** A 50% reduction in **cardiac output** would also decrease oxygen delivery, but the question asks for the factor that, when reduced by 50%, cuts oxygen delivery in half.
**Option C:** A 50% reduction in **oxygen saturation** would impact oxygen delivery, but its effect would be less pronounced compared to a reduction in hemoglobin.
**Option D:** This option is not provided, but typically, other factors like **2,3-bisphosphoglycerate** or **pH** would have a lesser impact on oxygen delivery compared to hemoglobin.

**Clinical Pearl / High-Yield Fact**
Remember, **hemoglobin** is a critical determinant of oxygen delivery, and its reduction can have severe clinical consequences, including tissue hypoxia.

**Correct Answer:** D. hemoglobin