Final answer:
RDW (red cell distribution width) measures the variation in size of red blood cells. A high RDW can indicate certain medical conditions such as anemia.
Step-by-step explanation:
The correct answer is D. Measures the variation in size of red blood cells.
RDW, or red cell distribution width, is a measure of the variation in size of red blood cells. A high RDW indicates that there is a wide range of sizes of red blood cells in the sample, which can be an indication of certain medical conditions such as anemia.
For example, in iron deficiency anemia, the red blood cells become smaller and more varied in size, resulting in an increase in RDW. On the other hand, in vitamin B12 deficiency anemia, the red blood cells become larger and more varied in size, also leading to an increase in RDW.