Final answer:
Reposition the patient to relieve pressure on the sacrum and promote blood circulation.
Step-by-step explanation:
The initial nursing action for an immobilized patient with erythema on the sacrum is c. Reposition the patient. This is important because prolonged pressure on the skin can lead to pressure ulcers or bedsores. By repositioning the patient regularly, it helps to relieve pressure on the affected area and promote blood circulation.