Answer:
b. Weber's law
Step-by-step explanation:
Weber's law states the in order for a person to experience a change in the intensity of a stimulus the difference between the initial and final exposure must be large.
Here, initially the light was at 25 watts and then was turned to its maximum. Later the light was at 90 watts then it was turned to its maximum. In the first case the difference between the initial and final stimulus is large and so my roomate was able to tell the difference. While in the later case the initial stimulus was already close to the maximum and did not notice when it was at the actual maximum.
Hence, my roommate did not notice that the light has increased.