Final answer:
The main source of fluids for an infant until about 12 months of age is formula or breast milk, which provides necessary hydration, nutrition, and immunity support.
Step-by-step explanation:
The main source of fluids for an infant until about 12 months of age is A) Formula or breast milk. The neonatal kidneys are still maturing and are not efficient at concentrating urine, which is why infants need to get enough fluids to stay hydrated. Breast milk and formula not only provide the necessary fluids but also deliver essential nutrients for growth and development, as well as immunoglobulins that help protect the child against infections. Colostrum, the first type of breast milk produced after birth, is particularly important as it boosts the newborn's immune system with a high concentration of antibodies.
As the baby grows, breast milk evolves to meet their changing nutritional needs. Foremilk comes first during a feeding, providing hydration, while the hindmilk, rich in fat, helps satisfy the infant's appetite. This dynamic nature of breast milk ensures the infant receives both proper nutrition and adequate hydration.