Final answer:
The best response for the nurse to give a parent about contacting the physician regarding an infant with diarrhea is option B) The pediatrician should be contacted if the infant has two loose stools in an 8-hour period.
Step-by-step explanation:
The best response for the nurse to give a parent about contacting the physician regarding an infant with diarrhea is option B) The pediatrician should be contacted if the infant has two loose stools in an 8-hour period. This is because frequent loose stools in a short period of time can lead to dehydration, which can be more serious in infants. It is important to monitor the frequency and consistency of the infant's bowel movements and seek medical advice if necessary.