**Core Concept**
Human milk is specially adapted to meet the nutritional needs of infants, differing from cow's milk in several key aspects. The composition of human milk varies throughout lactation, with changes in fat, protein, and carbohydrate content.

**Why the Correct Answer is Right**
Human milk contains more lactose compared to cow's milk. This is because lactose serves as a primary source of energy for infants. The high lactose content in human milk also facilitates the growth of beneficial gut microbiota in infants. Additionally, human milk contains a unique combination of oligosaccharides, which act as prebiotics, promoting the growth of beneficial bacteria in the gut.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because human milk contains less protein compared to cow's milk. The protein content in human milk is lower, but the proteins are more easily digested by infants.
**Option B:** This option is incorrect because human milk contains more water compared to cow's milk, but this is not the most significant difference.
**Option C:** This option is incorrect because human milk contains less fat-soluble vitamins, but this is not the primary difference between human milk and cow's milk.

**Clinical Pearl / High-Yield Fact**
Human milk is dynamic and adapts to the changing needs of the infant throughout lactation. The composition of human milk is influenced by factors such as the mother's diet, gestational age, and lactation stage.

**Correct Answer: D. Lactose.