The answer is c. nephron, villi, alveoli.
Capillaries are the smallest of all blood vessels with very narrow diameters (5-10 micrometres) and thin walls. Capillaries are very close to the cells in different organs and their thin walls enable an exchange of molecules between the cells. So, to found out which of the structures rely on capillaries, we must look for the smallest functional units of the tissues which are small enough to communicate with capillaries and, as well, to be the functional units:
a, b, d. none of those is the smallest functional unit, some are cells, tissues, organs.
c. nephron is the basic functional unit of kidneys, villi are the smallest functional units of the intestine, and alveoli are the smallest functional units of the lungs.
