**Core Concept**
The biological value of a protein measures its ability to provide essential amino acids for the body, supporting growth and maintenance. It is an indicator of the protein's quality and its capacity to be utilized by the body for various functions, such as muscle synthesis and repair.

**Why the Correct Answer is Right**
The correct answer is casein due to its slow digestion rate, which allows for a sustained release of amino acids into the bloodstream. This characteristic enables the body to utilize the essential amino acids from casein more efficiently, resulting in a higher biological value. Casein's slow digestion is attributed to its micelle structure, which protects the protein from digestive enzymes, thus prolonging its availability for absorption.

**Why Each Wrong Option is Incorrect**
**Option A:** Whey protein has a faster digestion rate compared to casein, leading to a rapid increase in amino acid levels in the bloodstream, but this rapid spike is followed by a decline, resulting in a lower biological value.
**Option B:** Egg protein has a high biological value, but it is not the highest among the options provided.
**Option C:** Soy protein is a plant-based protein with a moderate biological value, but it is not the highest among the options provided.

**Clinical Pearl / High-Yield Fact**
When selecting a protein supplement, consider the protein's biological value and its digestion rate, as these factors can impact the body's ability to utilize the protein for optimal growth and maintenance.

**Correct Answer:** C. Casein.