Final answer:
To ensure effective teaching on how to care for a newborn's umbilical cord, the parent should state that they will avoid giving the baby a sponge bath until the cord falls off.
Step-by-step explanation:
The nurse determines teaching is effective if the parent makes the statement: 'I will avoid giving my baby a sponge bath until the cord falls off.' This is the correct statement because it is important to avoid submerging the umbilical cord stump in water until it falls off and the navel heals completely. The other options, such as cleaning the cord stump with alcohol after each diaper change, covering it with a bandage, or folding down the diaper, are incorrect because they can hinder the healing process and increase the risk of infection.