"much, more, the most"
Translation:multe, pli multe, la plej multe
I'm not 100% sure on this, but I think the idea is that when a descriptive word is used, and the object being modified by said descriptive word is inferred, but not actually present (or when a word is being used alone, either as an interjection, or as an example, like it is here), it's more logical for it to be in adverbial form, since there isn't actually an object present in the statement for an adjective to modify.....
But this exact sentence is used in this lesson with bona and granda, e.g. "Bona, pli bona, la plej bona," and "Granda, pli granda, la plej granda." I don't see how these are any less exemplary or their objects any less implied. The course authors seem to see an inherent difference in usage between these words and multe in this case.
Hmm, then i think the authors had it in mind when they made this one that they were modifying a verb, otherwise it would be "Multaj, pli multaj, la plej multaj." On my previous statement, I may not have been completely accurate. I have seen a few debates before, about when a descriptive word is used as an interjection, whether or not it should be adverbial, regardless of whether it is modifying a noun or verb. I think when I saw this I overthought it, because that was the first thing that popped into my head, not the more logical idea that they were simply trying to infer a verb being modified, instead of a noun.....
So you are saying the idea is that they are implying something like "[He doth protest] much, [he doth protest] more, [he doth protest] the most?" I guess that would make sense, but I have to think that they would have thought to include "Multaj, pli multaj, la plej multaj" as an alternative answer if it were common since it is so much more consistent with the other sentences. I think there is some weirdness here that we need an expert to resolve.