**Core Concept:**
The question is related to the cyanosis, which is a condition where the skin and mucous membranes appear bluish due to decreased oxygenation of blood. Cyanosis occurs when the concentration of deoxygenated hemoglobin is increased. Hemoglobin consists of two main forms: oxyhemoglobin (oxygen-bound) and deoxyhemoglobin (oxygen-unbound). The normal oxygen saturation in arterial blood is 97-100%.

**Why the Correct Answer is Right:**
The correct answer, **D**, refers to the minimum concentration of reduced hemoglobin, which is the term for deoxygenated hemoglobin. Cyanosis develops when the proportion of deoxygenated hemoglobin in the blood increases. Reduced hemoglobin carries less oxygen than oxyhemoglobin, leading to decreased oxygen delivery to tissues and the characteristic blue discoloration.

**Why Each Wrong Option is Incorrect:**
A. Reduced hemoglobin is incorrect because the question is asking for the minimum concentration of reduced hemoglobin, not total hemoglobin concentration.
B. Oxygen saturation is not the correct answer because cyanosis results from the presence of deoxygenated hemoglobin, not the saturation level of oxygen in the blood.
C. Oxyhemoglobin is incorrect because cyanosis occurs when the proportion of deoxygenated hemoglobin increases, not the total concentration of oxyhemoglobin.

**Clinical Pearl:**
Understanding the relationship between hemoglobin forms and cyanosis is crucial for recognizing and diagnosing this condition. Adequate oxygenation of blood is essential for maintaining normal tissue function, and cyanosis can indicate severe respiratory or circulatory disorders, such as respiratory insufficiency, respiratory distress, or cardiac failure.

**Correct Answer:** .