Final answer:
The nurse's observation that warrants additional instruction is the baby's cheeks puffing in and out during feeding, which indicates a poor latch and potential feeding difficulties.
Step-by-step explanation:
When observing a newborn being bottle-fed, certain behaviors may indicate the necessity for additional teaching about proper feeding techniques. The observation by the nurse that requires more teaching is (d) the baby's cheeks puff in and out during feeding. This is because a correct latch should result in the cheeks staying rounded and not puffing, which suggests that the baby is struggling to form an effective seal around the bottle's nipple and may be taking in air, potentially leading to gas or discomfort. The mother holding the bottle horizontally (a) and the baby having rhythmic sucking and swallowing (c) are normal and desirable behaviors. The baby's lips flanging outward (b) is also a sign of a good latch on the bottle's nipple.