C is best. 1)两个句子平行，意思连贯；2) it明确地指代the Quetzalcoatlus；3) 根据句意应该用完成时态believed to have been，句末的has ever seen是个暗示, to have been时态上与后面的the world has ever seen对应。
A, 错在believed to be修饰对象模糊，逗号+done做定语优先修饰前面就近的名词。参考大全174：
Elephants emit low-frequency sounds, believed to originate from a small area on their foreheads, that they may use as a secret language to communicate with other members of the herd.
主语+谓语+宾语，done。如果过去分词放在句末是修饰主语的，可以认为这个句型不是ETS喜欢的。
另外believed to be有Quetzalcoatlus现在仍存在的意味。to have been比to be好，to have been表believe之前就是的。
B中and that在这里that指带不对。Gmat中that一般不单独用来指代某物。
D, E的which一般只能指前面的名词wingspan，错。
补注：done或是形容词短语，放在句末，多是修饰前面的名词，而非首先修饰句首主语。当然在没有引起歧义的情况下，有时也指向主语。在前面有名词(a wingspan)的情况下，believed to be尽量不放在句末为好，也许the Quetzalcoatlus (believed to be)had a wingspan这样放置更清楚些。这题主要还是to be the largest flying意思表达不准确。
一个老外点评：This is a badly-written sentence, without a correct answer. In my understanding, the correct answer should read "and is believed to be (or have been) the largest flying creature the world has ever seen. However, I will default to the second best answer, which is C | 
