**Core Concept**
Net Protein Utilization (NPU) is a measure of the efficiency with which the body can utilize dietary protein for growth, maintenance, and repair. It takes into account the protein's amino acid composition, digestibility, and absorption. NPU values range from 0 (no utilization) to 100% (complete utilization).

**Why the Correct Answer is Right**
The correct answer is related to the amino acid composition of the protein. Casein, a milk protein, has a high NPU value due to its rich content of essential amino acids, particularly branched-chain amino acids (leucine, isoleucine, and valine). The body can efficiently utilize casein's amino acids for various metabolic processes, resulting in a maximum NPU of 100%. This is because casein's amino acid profile closely matches the body's requirements, making it an ideal protein source.

**Why Each Wrong Option is Incorrect**
**Option A:** Whey protein, although highly digestible, has a slightly lower NPU value compared to casein due to its lower content of branched-chain amino acids.
**Option B:** Soy protein, a plant-based protein, has a lower NPU value than casein due to its lower content of essential amino acids, particularly methionine.
**Option C:** Egg protein, while highly valuable, has a slightly lower NPU value than casein due to its slightly lower digestibility and content of certain essential amino acids.

**Clinical Pearl / High-Yield Fact**
When evaluating protein sources for patients with malnutrition or muscle wasting, it's essential to consider the NPU value of the protein to ensure optimal utilization and recovery.

**Correct Answer: B. Casein.**