Final answer:
A mother can consider introducing a cup to her breastfeeding baby at around 6 months of age, when the infant has developed sufficient motor skills and visual acuity to handle a cup.
Step-by-step explanation:
The correct answer to the question regarding when a mother understands she can introduce a cup to her breastfeeding newborn is around 6 months of age. At this stage, infants develop the ability to pick up objects, transfer them from hand to hand, and sit upright, which are key skills needed to handle drinking from a cup. Moreover, their vision is nearly as acute as an adult's and they begin to understand their environment better, enjoy vocal play, and may make two-syllable sounds. Introducing a cup at about 6 months can facilitate the transition from exclusive breastfeeding as the infant starts to develop the necessary motor skills to use a cup.