Hi annie63!
Happy to address answer choice (D). First, note the three elements covered in the correct answer (B):
Cecilia Payne [1] proposed an innovative alternative to the view of the sun's composition that prevailed in her time, but, although [2] her view was ultimately vindicated, [3] other scientists were slow to accept her claim.
In other words, this answer choice covers her innovativeness, the slowness of the scientific community to accept her view, and her ultimate vindication.
By contrast, answer choice (D) states:
Cecilia Payne was the first scientist to demonstrate that the sun contains both hydrogen and helium, but her claim initially encountered widespread resistance in the scientific community.
At first blush, this looks very similar to answer choice (B), as they both cover Payne's innovativeness and the resistance of the scientific community to her view. However, this answer choice fails to cover the entire passage by failing to mention Payne's ultimate vindication as well. In this way, it covers the information from the first two paragraphs, but it fails to account for the final paragraph.
Lastly, in general, this is a very common thing to see on main point questions! It's likely that several or even all of the incorrect answer choices will correctly summarize one or another portion of the passage, but will ultimately be incorrect because the answer choice fails to account for
all of the passage. Such answer choices might have been correct if the question was solely a must be true question, but when one is asked for the main point, it's essential to confirm that the answer choice indeed covers all of the paragraphs and not only some of them.