Step 1: Understanding the composition of mother's milk.
Mother's milk is considered the ideal food for infants because it provides essential nutrients required for growth and development.
Step 2: Nutrients present in breast milk.
Breast milk contains proteins, fats, carbohydrates, vitamins such as vitamin A, and minerals such as calcium and potassium which support the baby's development.
Step 3: Identifying the missing nutrient.
Iron is present in only a very small quantity in breast milk and is considered insufficient compared to other nutrients. Therefore, infants later require iron-rich complementary foods.
Step 4: Conclusion.
Thus, among the given options, iron is considered the nutrient that is not adequately found in mother's milk.
Final Answer:} Iron.