Juxtaglomerular cells secrete 'A' when there is reabsorption in 'B' ion concentration.
Choose the correct pair labelled as A and B respectively:

Correct option is C

Correct Answer: (c)
Explanation:
Juxtaglomerular (JG) cells release renin in response to a decrease in Na⁺ (sensed indirectly via macula densa detecting low NaCl delivery) or decreased renal perfusion. Thus the correct pair is Renin — Sodium.
Information Booster:
Renin initiates the renin–angiotensin–aldosterone system (RAAS) to restore blood pressure and sodium balance.
Macula densa senses low NaCl at the distal tubule → signals JG cells to release renin.
Renin converts angiotensinogen → angiotensin I → (ACE) → angiotensin II, causing vasoconstriction and aldosterone release (→ Na⁺ retention).
Aldosterone increases Na⁺ reabsorption in the distal nephron.
JG cells respond also to decreased renal perfusion pressure and sympathetic stimulation.
Additional Information (Incorrect Options):
(a) Carbonic anhydrase is an enzyme involved in acid–base and CO₂/HCO₃⁻ handling, not the JG renin response.
(b) ATPase, Potassium — Na⁺/K⁺-ATPase pumps are widespread in nephron but are not secreted by JG cells; K⁺ is not the primary trigger for renin release.
(d) Renin, Chloride — chloride is part of the macula densa signal (Cl⁻/NaCl flux), but classical pairing for exam items is renin with sodium (option c).