Final answer:
The letter that indicates the vasa recta is D. Therefore, the correct option is D.
Step-by-step explanation:
The correct letter indicating the vasa recta is D. The vasa recta are essential components of the kidney's vascular system, branching from the efferent arterioles and running parallel to the loops of Henle. These vessels are continuous with the peritubular capillaries, collectively forming a portal system.
Integral to renal function, the vasa recta play a crucial role in countercurrent exchange. This process facilitates the efficient reabsorption of solutes and water in the kidney, contributing to the regulation of fluid balance.
The specific positioning and structure of the vasa recta contribute to their ability to maintain osmotic gradients, ensuring the effective concentration and dilution of urine in the nephrons.
Therefore, the correct option is D.