Final answer:
The Weber fraction (k) for touch (heaviness) being 0.02 means that the just noticeable difference for touch (heaviness) is 2% of the original weight, supporting Weber's law which relates to sensory perception and psychophysical scaling.
Step-by-step explanation:
The Weber fraction (k) for touch (heaviness) is 0.02. This fraction tells us about the just noticeable difference (jnd), which is the smallest amount of change in a stimulus that can be detected as a different sensation. In the context of the question, it indicates that a weight needs to change by 2% from its original value before the difference is noticeable. This means option 2 is correct: The just noticeable difference for touch (heaviness) is 0.02. Weber's law states that the difference threshold is a constant fraction of the original stimulus which directly applies to this concept of the Weber fraction.
The study of just noticeable difference and sensory thresholds, including experiments on receptor density estimation with two-point discrimination tests and research into subliminal messaging, all contribute to our understanding of sensory perception and psychophysical scaling.