I can suggest a couple reasons why answer choice (D) is incorrect.
To my understanding, both the Scar Art and Native Soil movements consisted of forms of realist art
In looking back to the passage, I did find support for the view that Scar Art is a form of realism. For example, lines 44-45 refer to "the Scar artists’ radical approach to realism." However, I wasn't able to find support for the view that the Native Soil movement was necessarily a form of realism. To the contrary, lines 55-56 state, "Native Soil painters reacted instead by idealizing
traditional peasant life." Idealization seems to pull the Native Soil movement away from counting as realism, and I couldn't otherwise find direct support that explicitly identified it as a realist movement.
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.
Here, answer choice (D) seems to cover the first two paragraphs, which cover realism, but it fails to cover the final paragraph on the Native Soil Movement. By contrast, answer choice (E), "The Unforeseen Artistic Legacy of China’s Cultural Revolution" is much broader in scope (it covers the Cultural Revolution in the first paragraph, and the subsequent developments of the Scar Art and Native Soil movements in the second and third paragraphs, so it covers the entire passage.