Answer: Weber's Law
Step-by-step explanation:
Weber's law states that alteration or difference occurring in stimulus that can be fairly noticeable defines constant ratio of the actual stimulus. It is used field of quantitative psychology.
According to the question,Weber law is being displayed when Charlie has the capability to detect small boxes weight difference sufficiently as just-noticeable but is not able to identify the weight of washing machine and dryer as it is not noticed by him in general way .