6. The first system to classify blood types is known as which of the following?
A-S-R system
B-D-G system
A-B-Osystem
C-O-V system

The first system to classify blood types is known as A-B-O system.

Option: D

Explanation:

The blood group system "ABO" is the categorizing of human blood centered on the hereditary characteristics of red blood cells means erythrocytes as measured by the presence or absence of A and B antigens on the surface of the red cells. Thus individuals may well have blood type A, type B, type O or type AB.

It was absent until 1900, when Karl Landsteiner established the concept at the Vienna University why some blood transfusions were effective while others were lethal. Landsteiner established the blood group mechanism ABO by combining each of his staff's red cells and serum.