Which of the following indicates significant fluid retention related to heart failure?

Significant fluid retention in heart failure is primarily indicated by the presence of peripheral edema. In heart failure, the heart's reduced ability to pump effectively can lead to increased pressures in the venous system, resulting in fluid leaking into surrounding tissues. This manifests as swelling, typically noticeable in the lower extremities such as the legs, ankles, and feet.

Peripheral edema is a key clinical sign that healthcare providers monitor when assessing heart failure patients. It reflects not only fluid overload but also the body's compensatory mechanisms in response to inadequate cardiac output.

In contrast, the other options do not align with the implications of fluid retention in heart failure. Decreased weight would generally suggest fluid loss, which is not associated with retention. Increased exercise tolerance and high energy levels are typically indicative of improved cardiac function rather than fluid overload, as heart failure commonly results in fatigue and reduced physical stamina.