**Core Concept**
The body maintains serum calcium levels through a complex interplay of absorption, excretion, and regulation by various hormones. Parathyroid hormone (PTH) plays a crucial role in maintaining calcium homeostasis by stimulating calcium absorption in the intestine, increasing calcium release from bone, and promoting calcium reabsorption in the kidneys.

**Why the Correct Answer is Right**
When serum calcium levels decrease, the parathyroid glands release PTH, which acts on the kidneys to increase calcium reabsorption and phosphate excretion. PTH also stimulates the release of calcitriol (1,25-dihydroxyvitamin D), the active form of vitamin D, which enhances calcium absorption in the intestine. To maintain serum calcium levels on a low calcium diet, the body increases PTH secretion, leading to increased calcitriol production and calcium absorption.

**Why Each Wrong Option is Incorrect**
**Option A:** Vitamin D supplementation would increase serum calcium levels, but it is not the primary response to a low calcium diet.
**Option B:** Increased phosphate excretion is a consequence of PTH action, but it is not a direct mechanism to maintain serum calcium levels.
**Option C:** Calcium supplementation would directly increase serum calcium levels, but the body's response to a low calcium diet involves hormonal regulation.

**Clinical Pearl / High-Yield Fact**
In patients on a low calcium diet, monitoring PTH levels can help identify those at risk of hypocalcemia. Elevated PTH levels can indicate the body's attempt to maintain calcium homeostasis.

**Correct Answer:** B. Increased PTH and calcitriol production.