Explain the compatility of blood groups during blood transfusion

the blood groups are (A ,B,AB,O)THE blood group AB is called the universal receiver because any other blood group can be compatible with this blood group this is due to the absence of anti-bodies in this typewhile blood group O is called the universal donor because it is compatible with all blood groups due to the absence of antigen (A and B) while blood group A receives blood from itself and O and donates blood to itself and AB the same thing goes with blood group B

Since we don't see a strand; i got answers off a sheet and the correct answer is "2".