Final answer:
Assisting the client with heat application and ROM exercises is the most important nursing intervention for a client diagnosed with rheumatoid arthritis.
Step-by-step explanation:
The most important nursing intervention for a client diagnosed with rheumatoid arthritis is to assist the client with heat application and ROM exercises. Heat application can help alleviate pain and stiffness in the joints, while range of motion (ROM) exercises can help improve joint flexibility and prevent further joint damage. These interventions can also promote blood flow and reduce inflammation in the affected joints.