The macula densa cells respond to
a. antidiuretic hormone
b. changes in solute content of the filtrate
c. changes in pressure in the tubule
d. aldosterone

B. changes in solute content of the filtrate

Explanation:

At the junction between the straight part and the contorted part of the distal tubule, the wall adjacent to the renal corpuscle changes: the cells are columnar, with central nuclei next to each other. This region is the macula dense. It monitors the Na + and Cl concentration of the filtrate and, through a paracrine signaling mechanism, informs the juxtaglomerular cells. They are sensitive to tubular flow and secrete vasoconstrictor or vasodilator substances that act on afferent arteriole.

Dense macula cells are detectors of flow velocity and liquid composition within the nephron at the end of the thick ascending limb. These detector cells contribute to the control of glomerular filtration rate and the control of renin secretion.

1 and 6 for apex