Why is A correct and not D?

I can see how one can be tricked by answer choice (D), however pay very close attention the words that the author uses. In line 26 she states the fact that most of Byron's poems cannot be convincingly read as subtle verbal creations indicates that Byron is not a "great" poet. She doesn't say that this is evidence that Byron is most definitely not "great," merely that it "indicates," i.e. suggests, that he is not. We are looking for the question that the passage gives a "DEFINITIVE" answer to. Further, in lines 31-32, she states "IF Byron is not a 'great poet," which points to the fact that it is possible he is not a great poet. On the other hand, answer choice (A) is definitively answered in lines 35-40: "Our main pleasure in reading Byron's poetry is the contact with a singular personality. Reading his work gives us illumination - self understanding - after we have seen our weaknesses and aspirations mirrored in the personality we usually find in the poems." Hope that clears things up! Please let us know if you have any other questions.