Final answer:
The advised method to warm expressed breast milk for a babysitter is to place the bottle in a bowl of lukewarm water, as it is a safe technique for ensuring even warming without damaging the milk's nutritional content.
Step-by-step explanation:
The practical nurse should advise Mrs. Wasson to instruct the babysitter to use method (c): Place the bottle in a bowl of lukewarm water to warm the expressed breast milk. This method is safe and effective as it helps to evenly distribute the heat without the risk of overheating the milk, which can destroy the nutrients and antibodies in breast milk. Microwaving breast milk can create hotspots and is not recommended, and the milk does not have to be heated on the stove which could be time-consuming and less safe. Using breast milk directly from the refrigerator is not advisable as cold milk might not be well accepted by the infant and can upset their stomach.