Final answer:
Cord prolapse is the term used when fetal vessels/umbilical cord are over the cervical os. It is a serious obstetric emergency that requires immediate medical attention to prevent harm to the baby.
Step-by-step explanation:
The term used when fetal vessels/umbilical cord are over the cervical os is B) Cord prolapse. Cord prolapse occurs when the umbilical cord exits the uterus before the baby, which can compress the cord and cut off blood flow to the baby. It is a serious obstetric emergency that requires immediate medical attention to prevent harm to the baby.