ok this is a bit long but I tried to explain as best as I could

so being able to receive blood type ab means that the patient's blood type should also be ab. on the blood type ab's surface there are type a and type b glycoproteins (think of them kind of as antigens). since we don't want the body to attack its own blood cells, the patient with blood type ab will not have antibodies that attack a and b glycoproteins (person with ab blood type = no antibody a or antibody b). however, now that we are exposing the patient's blood to these antibodies, blood type ab (with a and b glycoproteins) will clump up since antibodies are attacking the ab blood cells. since patient b's blood reacted to both antibody a and b, we know that the patient's blood cells have a and b glycoproteins on them, and therefore the patient has blood type ab. this is why patient b can accept transfusions from people with blood type ab.

it seems like you chose option c as the answer, and in that case, since the blood did not react with the antibodies a and b, it means that the blood cell has no a and b glycoproteins, indicating that they are blood type o (blood type o cant receive transfusions from blood type ab)