The organ primarily responsible for the conversion of vitamin D into its active form is the kidneys. When vitamin D is obtained either from the skin (through sunlight exposure) or from dietary sources, it first undergoes hydroxylation in the liver to form 25-hydroxyvitamin D (calcidiol), which is the major circulating form. However, this form is still inactive.

The critical step for activation occurs in the kidneys, where the enzyme 1α-hydroxylase converts 25-hydroxyvitamin D into 1,25-dihydroxyvitamin D (calcitriol), the biologically active form of vitamin D. This active form is essential for regulating calcium and phosphate metabolism, promoting absorption of calcium in the intestines, and maintaining bone health.

To summarize:

- Vitamin D is initially hydroxylated in the liver to 25-hydroxyvitamin D.

- The kidneys carry out the second hydroxylation step, catalyzed by 1α-hydroxylase, forming active 1,25-dihydroxyvitamin D.

- This renal conversion is tightly regulated by factors such as parathyroid hormone (PTH), calcium, and phosphate levels.

Therefore, the correct answer is Kidneys.

Reference: Guyton and Hall Textbook of Medical Physiology, 14th Edition, Chapter 76 - Calcium, Phosphorus, and Magnesium Metabolism