Final answer:
In the case of a client in the third trimester of pregnancy experiencing painless vaginal bleeding and suspected placenta previa, the nurse should prepare for an emergency cesarean section.
Step-by-step explanation:
In the case of a client in the third trimester of pregnancy experiencing painless vaginal bleeding and suspected placenta previa, the nurse should prepare for an emergency cesarean section (option b). Placenta previa occurs when the placenta partially or completely covers the cervix, obstructing the birth canal. This condition can lead to severe bleeding during labor, which may put both the mother and baby at risk.