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Renin is an enzyme that plays an important role in the regulation of blood pressure. It is released by the kidneys when the sodium level drops or the blood pressure is low initiating the renin-angiotensin-aldosterone pathway.

The juxtaglomerular apparatus consists of the macula densa cells and the juxtaglomerular cells. The macula densa cells present in the distal convoluted tubule sense a decrease in the sodium levels or blood volume and signal the juxtaglomerular cells to produce renin and release it into the bloodstream. The renin acts in conjunction with aldosterone and angiotensin leading to constriction of blood vessels and increased sodium reabsorption. This increases the blood volume and blood pressure. Hence, option a is correct.

The juxtaglomerular apparatus is located in the nephron which is the basic unit of the kidney. Therefore, it can be said that renin is produced in the kidney. Hence, option b is correct.

The juxtaglomerular apparatus is located in a special part of the nephron where the distal convoluted tubule comes in contact with the glomerulus. Hence, it can be said that renin is produced in the nephron. Hence, option c is correct.

Renin is produced in the juxtaglomerular apparatus which is located in the nephron, a part of the kidney. Hence, option d is correct.