The term `Rh factor` refers to Option 1: Rhesus factor. This refers to the presence or absence of a specific antigen on the surface of red blood cells. The Rh factor is named after the Rhesus macaque monkey, as this antigen was first discovered in their blood. The Rh factor is important in blood transfusions and during pregnancy, as Rh-negative individuals can have an immune reaction if they receive Rh-positive blood or if an Rh-positive fetus is carried by an Rh-negative mother. The presence of the Rh factor is denoted as Rh-positive, while its absence is denoted as Rh-negative. It is important for individuals to know their Rh factor for medical purposes, and blood banks carefully match Rh types to ensure compatibility during transfusions.