Answer: A. antigen
Step-by-step explanation:
An antigen is a protein that is present over the surface of the red blood cells. The type of antigen present over the surface of the red blood cell determine the blood type ( A, B, AB, O). The Rh factor is an antigen which is an inherited protein that can be found over the surface of the red blood cells. If an individual posses Rh antigen in the blood that will exhibit the Rh positive blood type. The one which does not posses the Rh antigen in the blood will be consider as Rh negative blood type.