Hi Henry!
I'm not Robert, but I will certainly do my best to answer your questions
Quickly, regarding the word "essential," you make a good point about interpreting the word to mean one (of potentially many) essential factor(s). However, this depends on the context! The way the statistician's argument is phrased is very different from something like "X is essential for Y." The statistician uses "essentially" in this stimulus in a way that is much more akin to what Robert said.
Alright, on to the good stuff! Let's unpack the Meteorologist's response. After a quick glance, we can immediately spot the conclusion: they disagree with the Statistician. But why? Well, the Meteorologist says that they disagree because "any professional meteorologist will tell you that ... no significant aspect can be controlled by a single variable." The Meteorologist doesn't do any
of their own analysis, all they do is say that they disagree because professional meteorologists (an authority) disagree.
With this in mind, let's discuss more about answer choice (A). I'm going to take a slightly different approach on this one, since it looks like you've already reviewed some of the other explanations on here (though it's important to note that those explanations are wonderful and very accurate)! (A) describes the flaw as rejecting a partial explanation only because it is not complete, not because it is incorrect. But, does the Meteorologist really do that? They disagreed entirely with the Statistician, they didn't put any qualifiers on it (for example, something like "while you're correct that luminosity might influence land temperature... any professional will tell you that the luminosity cannot be controlled by only a single variable). For this reason (and the others described by different folks), we can eliminate (A)!
In terms of the appeal to authority you mentioned in (E), I think perhaps some of the confusion arises from which person is actually saying which statement. The Meteorologist says that "any professional will tell you that... no significant aspect can be controlled by a single variable." This statement is not the Meteorologist's from the stimulus, rather, it is what the Meteorologist says professional meteorologists will assert. So, we can see that the Meteorologist's disagreement stems from the current views of the authority figures in a specific scientific field. Additionally, we can see how this is a huge oversight on the Meteorologist's behalf, since the Statistician already accounted for it! This has been mentioned by others, but I think it's worth mentioning again, since it truly exemplifies why (E) is the right answer: not only does the Meteorologist appeal to authority, but they also fail to evaluate the counterexample to what meteorologists currently accept!
I hope this makes sense, feel free to ask any follow up questions if it doesn't
Kate