Concept:Universal donors are safe because their red blood cells lack antigens that can trigger a reaction in the recipient.Explanation:People with blood group O possess neither A nor B antigens on their red blood cells.When O blood is transfused, these red cells carry no surface markers for the recipient’s antibodies to attack.As a result, group O blood can be given to individuals of any ABO blood group without causing agglutination.Consequently, group O individuals are described as universal donors.Answer:A. antigens are absent in their blood cells