**Core Concept**
The biological value of proteins is a measure of their quality, reflecting how efficiently the human body can utilize them for growth and maintenance. It is an essential concept in biochemistry, particularly in the context of nutrition and metabolism.

**Why the Correct Answer is Right**
The proteins efficiency ratio (PER) measures the biological value of a protein source by comparing the weight gain of subjects consuming the protein with those consuming casein, a high-quality protein. This ratio takes into account the protein's ability to promote growth, repair tissues, and maintain overall health. The PER is calculated as the ratio of weight gain in subjects consuming the test protein to the weight gain in those consuming casein.

**Why Each Wrong Option is Incorrect**
**Option B:** The biological value refers to the proportion of absorbed nitrogen from a protein that is retained by the body, but it does not directly measure the efficiency of protein utilization.

**Option C:** The protein energy ratio is a measure of the energy yield from a protein source, which is not directly related to its biological value.

**Option D:** Calorific value refers to the energy content of a substance, which is not relevant to the biological value of proteins.

**Clinical Pearl / High-Yield Fact**
When evaluating the quality of a protein source, it's essential to consider its biological value, which can be estimated using the protein efficiency ratio (PER). A high PER indicates that the protein is of high quality and can be efficiently utilized by the body.

**✓ Correct Answer: A. Proteins efficiency ratio**