Explanation
Correct answer: D.
Choice D is the best answer because it most effectively illustrates the claim that the speaker has contradictory feelings while experiencing the sights and sounds of spring. This quotation indicates that the speaker is reclined in a grove listening to a thousand sounds. Even though the speaker is in a "sweet mood" and thinking "pleasant thoughts," those pleasant thoughts also bring to mind "sad thoughts." In other words, these lines illustrate the claim that the speaker is having contradictory thoughts while immersed in the sights and sounds of spring.
Why the other choices are wrong
Choice A
Choice A is incorrect. Although this quotation refers to several flowers (primroses and periwinkles) and indicates that the speaker is in a "bower," or shady spot among the trees—details which suggest that the speaker is experiencing the sights of spring—it doesn’t suggest that the speaker is having contradictory feelings, only that the speaker believes that the flowers are experiencing enjoyment.
Choice B
Choice B is incorrect. Although this quotation focuses on the sights of spring—namely, new leaves on nearby trees appear to be opening up ("The budding twigs spread out their fan") to feel the breeze—the quotation doesn’t suggest that the speaker feels conflicted about this: the statement "And I must think, do all I can" suggests the speaker’s determination to attribute feelings of pleasure to the trees, not that the speaker is experiencing contradictory feelings.
Choice C
Choice C is incorrect. Although this quotation indicates that the speaker isn’t certain what the birds are thinking ("Their thoughts I cannot measure"), there’s nothing to suggest that the speaker is experiencing contradictory feelings. Rather, the quotation suggests that although the speaker is uncertain about the birds’ feelings, the speaker believes that the birds’ movements likely suggest their pleasure.