Step 1: Understanding renin secretion.
Renin is a hormone-enzyme involved in the regulation of blood pressure, blood volume, and sodium balance in the body.
Step 2: Source of renin.
Renin is secreted by the juxtaglomerular (JG) cells present in the walls of the afferent arteriole of the nephron. These cells collectively form the JG apparatus.
Step 3: Conclusion.
Since renin is secreted by the JG cells, the correct answer is (A) JG apparatus.