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

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.

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