Final answer:
Dyspnea on exertion is the early symptom of heart failure in an older adult client, indicating the heart's inability to meet increased oxygen demand during physical activities.
Step-by-step explanation:
The early symptom of heart failure in an older adult client that may be indicated by the nurse's findings is D. Dyspnea on exertion. Dyspnea on exertion means that the patient experiences breathlessness or shortness of breath when engaged in physical activity, which is a hallmark early sign of heart failure. Since the heart is struggling to pump blood efficiently, it cannot meet the increased oxygen demand during exertion, resulting in dyspnea.