正确答案：AD
解析：
The sentence is questioning whether art has any real strength. It offers up what is ostensibly a powerful portrait capturing the horrors of war. "...is not an incontrovertible...", however, suggests that Guernica is going to seem almost meek. (A) and (D) are both synonyms for meek.
(E) is a great trap, especially because of "seaside idyll." But that describes the light more than the picture. Also, there is no match for (E) amongst the answer choices.