Final answer:
The IV insertion site preferred for infants under 9 months old is the scalp vein, due to easy visualization, access, and its stability.
Step-by-step explanation:
The nurse explains that for stability of the IV insertion site in an infant younger than 9 months of age, the insertion site is the scalp vein. In infants, the scalp veins are often used because they are easy to visualize and access, and they provide a stable site for IV placement.
Scalp veins can be a preferable choice in pediatric care, particularly for infants, due to the limited mobility of the head, which helps reduce the risk of the IV becoming dislodged.