Thanks for the question! So the part of the passage we want to focus on is the beginning of the second paragraph, from lines 19 to 21. In those lines, we’re told that secondary substances undoubtedly first appear as a result of genetic mutations in individual plants. The language in (D) is pretty strong, but the word “undoubtedly” in this part of the passage is strong language that matches the language used in (D). Because secondary substances undoubtedly appear as a result of genetic mutations, it cannot be true that they appear in direct response to insects. The rest of the second paragraph, however, does describe how insects indirectly influence which particular secondary substances are present in a plant species based on the interaction between insects and plants.
Hope this helps! Feel free to ask any other questions that you might have.