**Core Concept**
Net protein utilization (NPU) is a measure of protein quality, reflecting the efficiency with which the body utilizes ingested protein for growth, maintenance, and repair. It is an important concept in nutrition science, particularly in the assessment of protein quality in various food sources.

**Why the Correct Answer is Right**
The correct answer, B, accurately defines NPU as the ratio between nitrogen retained by the body and total nitrogen intake multiplied by 100. This calculation reflects the body's ability to utilize protein from the diet. When the body retains more nitrogen from the diet, it implies efficient protein utilization. NPU values range from 0 (indicating no protein utilization) to 100 (indicating complete protein utilization). This ratio helps in assessing the quality of protein in various food sources, such as animal and plant-based proteins.

**Why Each Wrong Option is Incorrect**
**Option A:** This option incorrectly defines NPU as the ratio of energy from protein to total energy in the diet. While energy from protein is an important consideration in nutrition, NPU specifically focuses on protein utilization and retention.

**Option C:** This option is incorrect as it describes the concept of amino acid score, which is a measure of the availability of essential amino acids in a protein. It does not accurately represent NPU.

**Option D:** This option is misleading, as a high NPU indicates efficient protein utilization, not necessarily a high protein requirement in the diet. In fact, a high NPU suggests that the body can utilize protein effectively, potentially requiring lower amounts of protein for optimal function.

**Clinical Pearl / High-Yield Fact**
A high NPU is often associated with high-quality proteins, such as those found in eggs, dairy, and lean meats. In contrast, plant-based proteins, like legumes and grains, generally have lower NPU values due to lower digestibility and bioavailability of essential amino acids.

**✓ Correct Answer: B. It is the ratio between nitrogen retained by the body and total nitrogen intake multiplied by 100**