Final answer:
The nurse's most appropriate action in this situation is to apply supplementary oxygen by nasal cannula (option A).
Step-by-step explanation:
The nurse's most appropriate action in this situation is to apply supplementary oxygen by nasal cannula (option A). The patient is experiencing a sickle cell crisis, indicated by the fine crackles on lung auscultation. The presence of crackles suggests fluid in the lungs, which could be due to vaso-occlusive events or acute chest syndrome seen in patients with sickle cell disease. By applying supplementary oxygen, the nurse can help improve oxygenation and alleviate symptoms.