And it is believed to have been-first, try to find the error(s), if any, in the original sentence. we cannot use just the phrase believed to be, for it seems to refer to wingspan (and, grammatically, it does) rather than to quetzalcoatlus, which is the flying creature mentioned in the sentence.
to correct the mistake, we need to put after the second comma the coordinating conjunction and and use the personal pronoun it to refer back to the antecedent, quetzalcoatlus, the original subject. the pronoun it is needed because of the presence of the second comma, which is not underlined and therefore not replaceable. the relative pronoun that is inappropriate when placed after the second comma as it cannot replace quetzalcoatlus. doing so would be grammatically incorrect.
finally, if the relative pronoun which is placed just after the second comma, it can refer only to wingspan or, perhaps, to feet without any confusion. so, choice d and choice e are incorrect for yet another reason.
from the above discussion, every choice can be eliminated except choice c, the correct answer.
note: the past infinitive to have been is needed instead of the present infinitive to be because the quetzalcoatlus has died out.