**Core Concept**
The 1,25-hydroxylation of Vitamin-D is a crucial step in its activation, converting 25-hydroxyvitamin D into its biologically active form, calcitriol. This process is mediated by the enzyme 1-alpha-hydroxylase, which is specifically localized in the kidneys.

**Why the Correct Answer is Right**
The 1,25-hydroxylation of Vitamin-D occurs in the proximal convoluted tubules (PCT) of the nephrons within the kidneys. This is where the enzyme 1-alpha-hydroxylase, a cytochrome P450 enzyme, is expressed. The PCT is responsible for reabsorbing glucose, amino acids, and other essential nutrients from the filtrate back into the bloodstream. In addition to these functions, the PCT also plays a critical role in the activation of Vitamin-D through the 1,25-hydroxylation process. This activated form of Vitamin-D then plays a key role in regulating calcium and phosphate metabolism in the body.

**Why Each Wrong Option is Incorrect**
**Option A:** The collecting ducts are primarily involved in water reabsorption and electrolyte balance, and are not the site of Vitamin-D activation.
**Option B:** The glomerulus is the site where blood filtration occurs, and is not directly involved in the activation of Vitamin-D.
**Option D:** The distal convoluted tubules are involved in fine-tuning electrolyte balance and acid-base regulation, but are not the site of 1,25-hydroxylation of Vitamin-D.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the activation of Vitamin-D in the kidneys is tightly regulated by parathyroid hormone (PTH) and calcium levels, which helps to maintain calcium homeostasis in the body.

**✓ Correct Answer: C. Proximal convoluted tubules**