Is the answer "C" ? ... I am not sure whether it is the answer?

can sme one explain what it in C is reffering to

for me it is definitely reffering to wingspan and not Quetzalcoatlus

C is the best one among all 5. "it" would be problematic if both the Quetzalcoatlus and the wing span shared a quality that was being referred to, ie: and it is believed to have been the "largest" the world has even seen. In that case, was it the Quetza or the wingspan that was the largest? But in the question, logically the wing span cannot be a flying creature, so it traces back to Quetza.

ie: My cat had cancer, and it died last month. clearly cancer cannot die, so it traces back to cat.
on the other hand: My cat had a baby, and it likes to eat. Both the cat and the baby can like to eat, so it is ambiguious

A really good solution provided by the moderator.
Have a look at this link https://www.beatthegmat.com/tense-t27770.html