Final answer:
The cells in the kidney that monitor NaCl concentrations are the macula densa cells, which are part of the juxtaglomerular apparatus. Therefore, correct option is b.
Step-by-step explanation:
In the kidney, the cells that monitor NaCl concentrations are the macula densa cells. The macula densa cells are located within the juxtaglomerular apparatus (JGA) at the point where the distal convoluted tubule (DCT) comes into contact with the afferent arteriole.
These cells function to regulate blood flow and the glomerular filtration rate (GFR) by sensing changes in the Na+ concentration of fluid passing through the DCT.
When there are variations in blood volume, blood pressure, or electrolyte balance, the macula densa cells communicate with the juxtaglomerular cells to adjust the GFR accordingly, either causing vasoconstriction or vasodilation of the afferent arterioles.