Final answer:
The umbilical line should be inserted 2-4 cm into the umbilical vein, just at the point of blood return.
Step-by-step explanation:
The umbilical line, also known as the umbilical catheter, is typically inserted into a newborn's umbilical vein for medical purposes such as administration of medications or fluids. The correct insertion point for the umbilical line is option A, which is 2-4 cm into the vein, just at the point of blood return.