Hi Katya,
Thanks for the question! there are two points here: 1. Could another person's opinion be a main point? and 2. Is that happening here? Let's look at both.
1. The answer here is yes, although obviously if the author states someone else's opinion and then supports it, they adopt it as their own. Something along the following lines would work: "Frank Jimenez, the eminent sociologist, has asserted a that people can only achieve what they envision. This opinion, while controversial, is strongly supported by..." In a case like this, one could say the other person's opinion is being supported as the main point.
2. The above aside, I don't think that's happening here. I don't read that first sentence as someone else's opinion. It's the scientist's opinion, and is stated rather flatly via that "There is little doubt..." opener. It seems like a series of factual assertions, with the first sentence being the umbrella that everything else falls under
Side note: Even if you do identify the first sentence as the main point, I love the tricky work they do with answer choice (A). It's a really attractive wrong answer, and it's easy to like it immediately and then not read all of the other answers completely, which leaves people prey to not realizing that (E) is the more accurate answer.
Please let me know if that above helps. Thanks!