Which blood group is universal donor?

**Core Concept**
The question is testing the understanding of blood group typing and compatibility, specifically in the context of transfusion medicine. The ABO blood group system is the primary classification system for human blood, which categorizes blood into four groups: A, B, AB, and O.

**Why the Correct Answer is Right**
The correct answer is the blood group that lacks both A and B antigens on the surface of red blood cells, making it compatible with all other blood groups. When transfused, Rh-negative blood is also a consideration, but in this context, we're focusing on the ABO blood group system. Individuals with the O blood group have no A or B antigens, making their blood suitable for transfusion to individuals with A, B, AB, or O blood groups. This is because the recipient's immune system will not recognize the O blood group as foreign, reducing the risk of an adverse reaction.

**Why Each Wrong Option is Incorrect**
* **Option A:** Individuals with A blood group have A antigens on their red blood cells, making their blood incompatible with B and AB blood groups.
* **Option B:** Similarly, individuals with B blood group have B antigens on their red blood cells, making their blood incompatible with A and AB blood groups.
* **Option C:** Individuals with AB blood group have both A and B antigens on their red blood cells, making their blood incompatible with O blood groups.

**Clinical Pearl / High-Yield Fact**
It's essential to note that the universal donor status of O blood group is only applicable when the blood is Rh-negative (O-). If the blood is Rh-positive (O+), it can be transfused to Rh-negative individuals, but not to Rh-positive individuals.

**Correct Answer:** D. O