**Core Concept**
Vitamin D therapy in Rickets is assessed by evaluating the normalization of serum calcium and phosphate levels, as well as the improvement in bone mineralization and density. This is crucial for preventing long-term complications of Rickets, such as growth retardation, skeletal deformities, and increased risk of osteoporosis.

**Why the Correct Answer is Right**
The correct answer assesses the effectiveness of Vitamin D therapy in Rickets by measuring serum levels of calcium and phosphate, which are essential for normal bone mineralization. **25-Hydroxyvitamin D** (25(OH)D) and **1,25-Dihydroxyvitamin D** (1,25(OH)2D) are the primary forms of Vitamin D that regulate calcium absorption and phosphate metabolism. By normalizing these serum levels, Vitamin D therapy helps restore normal bone growth and density.

**Why Each Wrong Option is Incorrect**
**Option A:** Serum alkaline phosphatase levels are not a direct measure of Vitamin D therapy efficacy in Rickets.
**Option B:** Urine calcium excretion is not a reliable indicator of Vitamin D therapy success in Rickets.
**Option C:** Bone density scans are not the primary method of assessing Vitamin D therapy efficacy in Rickets.

**Clinical Pearl / High-Yield Fact**
It's essential to monitor serum 25(OH)D levels regularly to ensure adequate Vitamin D supplementation in Rickets, as deficiencies can lead to treatment failure and long-term complications.

**Correct Answer:** D.