The most popularly known blood grouping is the $\mathrm{ABO}$ grouping. It is named $\mathrm{ABO}$ and not…
The most popularly known blood grouping is the $\mathrm{ABO}$ grouping. It is named $\mathrm{ABO}$ and not $\mathrm{ABC}$, because "O" in it refers to having :
No antigens A and B on RBCs
Other antigens besides A and B on RBCs
Overdominance of this type on the genes for $A$ and $B$ types
One antibody only-either anti-A and anti-B on the RBCs
Solution
The most well-known blood grouping is the ABO grouping, and it's named ABO because ""O"" in it refers to having no antigens A and B on red blood cells (RBCs).
In addition to the A and B antigens, there's also the Rh factor, which can be either present (+) or absent (–), creating the 8 most common blood types.