Final answer:
The described stage of heart failure is classified as Stage III, where patients are comfortable at rest but experience significant symptoms with mild activity.
Step-by-step explanation:
The stage of heart failure described in the question, where a marked limitation of physical activity is evident and less than ordinary activity causes fatigue, palpitations, or dyspnea, is classified as Stage III heart failure. At this stage, patients are comfortable at rest but experience significant symptoms with even mild exercise or activity. Managing heart failure often involves medications that help improve the heart's pumping ability and address specific symptoms, while lifestyle changes and sometimes procedures are necessary for long-term management of the condition.