Final answer:
The best indication that the newborn has achieved an effective attachment to the breast is when the baby's mouth covers most of the areolar surface. This position allows for efficient milk transfer and ensures the baby is properly latched onto the breast.
Step-by-step explanation:
The best indication that the newborn has achieved an effective attachment to the breast is B. Mouth covers most of the areolar surface. When the newborn's mouth covers most of the areolar surface, it ensures that the baby is properly latched onto the breast, allowing for efficient milk transfer. This position allows the baby to extract milk from the nipple and areola, triggering the let-down reflex and ensuring adequate nutrition for the newborn.