Final answer:
The RBC abnormality characterized by coinlike stacks of RBCs is called Rouleaux formation (option d), not to be confused with RBC agglutination or inclusions like Howell-Jolly bodies or Heinz bodies.
Step-by-step explanation:
The RBC abnormality characterized by coinlike stacks of RBCs is known as Rouleaux formation. This term refers to the phenomenon where red blood cells (RBCs) stack up in rows, resembling a stack of coins, which can be observed in a blood smear under a microscope. Rouleaux formation can be an indication of different conditions, including high levels of fibrinogen in the blood or inflammatory processes.
It is different from agglutination, which involves the clumping of RBCs due to the action of antibodies during hemagglutination, and from the presence of Howell-Jolly bodies or Heinz bodies, which are inclusions within the RBCs themselves and not a pattern of arrangement.