Final answer:
The index about the average size of red blood cells is A) Mean Corpuscular Volume (MCV), which is an important measurement for diagnosing different types of anemia based on RBC size.
Step-by-step explanation:
The MCV measures the average size of red blood cells (erythrocytes). RBCs are critical for delivering oxygen to the body tissues, and variations in their size can indicate different types of anemia or other health conditions. When RBCs are normal-sized, they are referred to as normocytic; when smaller than normal, they are called microcytic; and when larger than normal, they are termed macrocytic. The MCV is a valuable diagnostic tool, as it helps to differentiate the types of anemia based on the changes in RBC size.