Final answer:
Preparing for a cesarean section is the most appropriate action if labor is unable to be arrested.
Step-by-step explanation:
If labor is unable to be arrested, special actions in the intrapartum period should be taken. Among the options provided, the most appropriate action would be to prepare for a cesarean section. In some cases, labor may need to be stopped if it poses a risk to the mother or baby, and a cesarean section may be necessary to safely deliver the baby. Other actions listed, such as administering additional tocolytic drugs, encouraging the mother to push forcefully, or increasing the rate of oxytocin infusion, may not be appropriate or effective in this situation.