Final answer:
CPR is ineffective if the patient is in a prone position, which is lying face down. The patient needs to be on their back (supine) on a firm surface for CPR compressions to properly circulate blood.
Step-by-step explanation:
Cardiopulmonary resuscitation (CPR) is a life-saving emergency procedure that requires a patient to be positioned correctly for it to be effective. The correct answer to the multiple-choice question is B. prone. CPR is not effective when a patient is prone because this position means lying face down, and chest compressions need to be administered to the front of the chest while the patient lays on their back (supine) and on a firm surface. To properly perform CPR, the patient should be lying on a firm, flat surface in a supine position so that chest compressions can effectively pump blood from the heart into the circulation. In this position, rescuers can apply the correct force to the sternum between the T4 and T9 vertebrae, following the guidelines of at least 5 cm deep compressions at a rate of 100 compressions per minute.