What does a hematocrit test measure, and what is the purpose of the equipment shown in the image?

The correct answer is C.

A hematocrit test measures the percentage of red blood cells in a blood sample. It is the ratio of the volume of red blood cells to the total volume of blood.

The image shows a microcentrifuge, which is used to spin blood samples in capillary tubes (like the ones shown) to separate the blood components, allowing for the measurement of hematocrit.

The final answer is $\boxed{\text{C. Percentage of red blood cells}}$.