I'm not familiar from the passage, but from the information given in the explanation for the answer I feel like I could explain this. It seems like in the passage they described that they were testing for sensitivity to hot sauce, specifically to capsaicin (the ingredient in hot sauce that actives your thermoreceptors). The stimulus (amount of sauce) is given in units of SHU, and they separate the study participants based on how 'sensitive' they are to the hot sauce. For example, maybe they asked the participants to rate how hot each dosage of sauce was.
Now in order for the study to work, people have to be able to tell the difference between 1000 SHU sauce and 2500 SHU sauce.
Weber's Law basically says that, when exposed to a stimulus, you will only notice a change in the magnitude of the stimulus above a certain threshold. For example, if you are blindfolded and holding a 100 pound weight, you may not notice if someone adds another five pounds on top. You may not notice if someone adds another 9.9 lbs on top. However, if someone adds ten pounds on top, you may notice. In this example, the just noticeable difference is ten pounds.
Weber's Law is mathematically represented by:
∆I/I = k , where I is the magnitude of the original stimulus, ∆I is the JND, and k is weber's constant. Because k is a constant, this means that the ratio of the JND to the original stimulus is FIXED. This is important. In the example I set up with the muscle, the original stimulus (I) is 100lb, and the JND (∆I) is 10lb. This means that k = 0.1. So if you change the example to make it so that the original weight you're holding is 1000lb, the JND is no longer 10lb, now it is 100lb.
In the passage you're asking about, since the study has to make sure that people can distinguish between 1000 SHU and 2500 SHU, the JND (for an original stimulus of 1000 SHU) must be less than 1500 SHU. To be clear, it is a fixed number that we can't know specifically from the information given. If they had told us that participants noticed no difference between 1000 SHU and 1500 SHU, that means the JND must be above 500 SHU. Maybe it's 818, who knows.
The question is worded ambiguously, because it doesn't ask what the JND is relative to a specific stimulus, and to get the right answer you have to assume that they are referring to the lowest possible number. Because if they asked what the JND is relative to an initial test of 2500 SHU, the answer would be less than 2500 SHU (because the next dosage they used to show sensitivity was 5000 SHU). Hope that makes sense.
