Final answer:
The most consistent indicator of pain in infants is a facial expression of discomfort. While increased respirations or heart rate can also signify distress, they are not as specific to pain as facial expressions or crying.
Step-by-step explanation:
The most consistent indicator of pain in infants is D. a facial expression of discomfort. Infants are unable to verbally communicate their discomfort, but they can express it through facial expressions and crying. The Apgar score assesses five criteria: skin color, heart rate, reflex, muscle tone, and respiration. While increased heart rate and respirations can be indicators of pain or distress, they are also influenced by other factors. On the other hand, a facial expression of discomfort or crying is a more direct and specific indicator that an infant may be in pain.