Final answer:
The best intervention for a client with pneumonia who exhibits crackling lung sounds would be to have the client cough and deep breathe.
Step-by-step explanation:
The best intervention for a client with pneumonia who exhibits crackling lung sounds would be to have the client cough and deep breathe (option C). This intervention helps to clear mucus from the airways and promote better lung function. Incentive spirometer (option B) is also a useful intervention, but the priority is to have the client actively cough and deep breathe.