Final answer:
The most important reason for a client who recently had a modified radical mastectomy to exercise her arm is to prevent lymphedema and improve circulation.
Step-by-step explanation:
The most important reason for a client who recently had a modified radical mastectomy to exercise her arm is to prevent lymphedema and improve circulation (C). Lymphedema is a condition that causes swelling in the arm due to a build-up of lymph fluid, and it is a common complication after mastectomy. Regular exercise can help improve the flow of fluid and prevent the development of lymphedema. Furthermore, exercise also promotes healing of the surgical site and overall strength and endurance.