**Core Concept**
The whey/casein ratio in breast milk is a critical determinant of infant nutrition, influencing protein digestion, absorption, and utilization. A balanced whey/casein ratio supports optimal growth and development in infants.

**Why the Correct Answer is Right**
The correct answer, 60:40, reflects the average whey/casein ratio in human breast milk. This ratio is essential for promoting efficient protein digestion and absorption in infants. Whey proteins are more easily digested and absorbed, while casein proteins provide a slower, more sustained release of amino acids. This balance supports the growth and development of infants, particularly during the first few months of life.

**Why Each Wrong Option is Incorrect**
**Option B:** An 80:20 ratio would result in an excessive amount of casein, leading to poor protein digestion and absorption in infants.

**Option C:** The value 0.888888889 is incorrect and does not represent a valid whey/casein ratio in breast milk.

**Option D:** A 40:60 ratio would result in an excessive amount of casein, similar to Option B, leading to poor protein digestion and absorption in infants.

**Clinical Pearl / High-Yield Fact**
The whey/casein ratio in breast milk is a critical factor in infant nutrition, and a balanced ratio is essential for supporting optimal growth and development. This ratio should be maintained in infant formula to ensure that formula-fed infants receive similar nutritional benefits as breastfed infants.

**✓ Correct Answer: A. 60:40:00**