What could be the blood type of the unknown person in row one of the diagram?

it depends on your blood type there are many different kinds so it depends on if u are using certain ones or all of them and the number of people

It depends if you are A pos or A neg. If you are A pos, you can receive A pos, A neg, O pos, and O neg. If you are A neg, you can receive A neg, or O neg. The reason for this is that if you are A neg and receive Rh positive blood, it is possible you may develop an Anti-D antibody (Rh antibody) which would cause you to have a reaction to further transfusion of Rh positive units.