**Core Concept**
Breast milk composition is a critical aspect of lactation and infant nutrition. It is a dynamic fluid that changes in composition over time, providing the optimal mix of nutrients, immunoglobulins, and growth factors for infant development.

**Why the Correct Answer is Right**
The composition of breast milk is approximately 87% water, 4% carbohydrates (lactose), 3% proteins (casein and whey), and 1% fats (triglycerides). The exact composition can vary depending on factors such as the mother's diet, lactation stage, and individual differences. The unique blend of nutrients and immunoglobulins in breast milk supports infant growth, development, and immune system maturation.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because breast milk does not contain a significant amount of fiber.
**Option B:** This option is incorrect because while breast milk does contain some cholesterol, it is not the primary source of energy for infants.
**Option C:** This option is incorrect because breast milk is not primarily composed of proteins (only 3% of total composition).

**Clinical Pearl / High-Yield Fact**
Breast milk is often described as "living milk" due to its dynamic composition and the presence of immunoglobulins, which provide protection against infections and support immune system development in infants.

**Correct Answer:** D.