You are working out and you adjust the difficulty program on your elliptical machine. You raise the "slope" to make the workout harder. A psychophysicist would call the minimal amount you need to make this adjustment in order to detect a change in the intensity of the stimulus the __________.

A) decision criterion
B) false alarm
C) Weber fraction
D) just-noticeable difference

The just-noticeable difference (JND) is the minimal amount of change needed to detect a difference in the intensity of a stimulus. In this scenario, the change in the intensity of the stimulus is the increase in the slope of the elliptical machine, and the JND would be the smallest amount of increase in slope that the person can detect. Therefore, the correct answer is D, just-noticeable difference. Choice A (decision criterion) refers to the threshold at which a person decides whether a stimulus is present or absent. Choice B (false alarm) refers to mistakenly perceiving a stimulus as present when it is not. Choice C (Weber fraction) refers to the proportional change needed to detect a difference in the intensity of a stimulus, rather than the absolute amount.