Answer:
The second major detail in the paragraph is indeed introduced by c. sentence 4.
Step-by-step explanation:
This paragraph offers three major details about the claim. The claim is that chimpanzees are skillful tool-users. Then, the details mention how they can use three different objects in a creative, intelligent manner. The first detail talks about sticks, and is introduced by sentence 2. The second major detail talks about leaves, and is introduced by sentence 4. Finally, the third detail is about stones, and is introduced by sentence 6.
Therefore, to answer the question: the second major detail is introduced by sentence 4 -- 4) In addition, chimps use leaves in a variety of ingenious ways.