In a second-year class of nurse assistants, Sister Bonsa was teaching on the topic types of blood group? After her lecture she asked the students to recap what she thought. One student indicated that the universal blood donor is Blood group A. AB B. O- C. O+

A universal blood donor is a person whose blood can be transfused to almost any recipient, regardless of the recipient's blood type, without causing a severe immune reaction. This is because their red blood cells lack specific antigens (A, B, and Rh factor) that would trigger an immune response in the recipient.

• A. AB: Individuals with AB blood type have both A and B antigens on their red blood cells. They are considered universal recipients because they can receive blood from all other blood types, but they are not universal donors.
• B. O-: Individuals with O- blood type have neither A nor B antigens, and they also lack the Rh factor. This means their red blood cells do not carry any common antigens that would cause an immune reaction, making them the universal donor.
• C. O+: Individuals with O+ blood type lack A and B antigens but possess the Rh factor. While O+ blood can be given to all Rh-positive blood types, it cannot be given to Rh-negative individuals, so it is not a truly universal donor.

Based on the presence or absence of A, B, and Rh antigens, blood group O- is the universal donor.

The final answer is $\boxed{\text{B. O-}}$