**Core Concept**
The universal donor blood group is a term used to describe the blood group that can be transfused to recipients with any ABO blood group without causing a hemolytic reaction. This is due to the absence of A and B antigens on the surface of the red blood cells.

**Why the Correct Answer is Right**
Blood group O is the universal donor blood group because it lacks the A and B antigens on its red blood cells. This means that type O blood can be transfused to individuals with any ABO blood group (A, B, AB, or O) without causing an adverse reaction. The absence of A and B antigens on the surface of type O red blood cells prevents the recipient's immune system from recognizing them as foreign and attacking them.

**Why Each Wrong Option is Incorrect**
**Option A:** Type A blood contains the A antigen, which can trigger an immune response in individuals with type B or AB blood, making it not a universal donor.
**Option B:** Type B blood contains the B antigen, which can trigger an immune response in individuals with type A or AB blood, making it not a universal donor.
**Option C:** Type AB blood contains both A and B antigens, which can trigger an immune response in individuals with type O blood, making it not a universal donor.

**Clinical Pearl / High-Yield Fact**
The universal donor blood group (O) is often referred to as "O negative" or "universal donor" due to its ability to be transfused to recipients with any ABO blood group without causing a hemolytic reaction.

**✓ Correct Answer: D. O**