Question

term
absolute threshold
the detection of physical energy emitted or reflected by physical objects; it occurs when energy in the external environment or the body stimulates receptors in the sense organs
the absence of normal levels of sensory stimulation.
the smallest quantity of physical energy that can be reliably detected by an observer.
the smallest difference in stimulation that can be reliably detected by an observer when two stimuli are compared; also called just noticeable difference.

Explanation:

The absolute threshold in psychology is defined as the smallest quantity of physical energy that can be reliably detected by an observer. The first option describes general sensation detection, the second is about sensory deprivation, and the fourth is about the just - noticeable difference.

Answer:

The smallest quantity of physical energy that can be reliably detected by an observer.