**Core Concept:**
The question is testing our knowledge about blood types, specifically which blood type can donate blood to any other blood type without causing a reaction. The human blood group system is classified into four main groups: A, B, AB, and O. Each blood type is defined by the presence or absence of specific antigens on the surface of red blood cells.

**Why the Correct Answer is Right:**
The correct answer, **D. O positive**, is a universal donor because it lacks both A and B antigens and only has the Rh positivity. This means that a person with blood type O positive can donate blood to anyone, regardless of their blood type, without causing a reaction.

**Why Each Wrong Option is Incorrect:**
Option A (O negative) is incorrect because it lacks Rh positivity, which is essential for a universal donor. Option B (A positive) and C (AB positive) are incorrect because they both have antigens that could cause a reaction when receiving blood from a universal donor. Option D (B positive) is incorrect as it has antigen B, which can cause a reaction when receiving blood from a universal donor.

**Clinical Pearl / High-Yield Fact:**
Understanding blood types is crucial in transfusion medicine. A blood type mismatch between a donor and recipient can lead to hemolytic disease of the newborn, immune responses, or transfusion reactions in the recipient. Ensuring a compatible blood type is essential for safe and effective blood transfusion.

**Explanation:**
A universal donor is a blood type that can receive blood from any other blood type without causing a reaction. In this case, the correct answer is D. O positive. This is because it lacks antigens A and B and is Rh positive. Therefore, a person with blood type O positive can donate blood to anyone without causing a reaction.

**Correct Answer: D. O positive**

**Why Each Wrong Option is Incorrect:**

A. O negative: O negative lacks the Rh factor, rendering it incompatible with all individuals except those with O negative blood type.
B. A positive: Antigen A is present, which could lead to a reaction when receiving blood from a universal donor.
C. AB positive: Antigen B is present, which could lead to a reaction when receiving blood from a universal donor.
D. O positive: The Rh factor is present, making it compatible with all blood types except O negative.

**Why D is the Correct Answer:**
The presence of the Rh factor in O positive makes it compatible with all blood types except O negative. This means that an individual with blood type O positive can donate blood to anyone without causing a reaction, making it a universal donor.