Answer:
The sentence that best states the main idea of the text is indeed the third one: I have always wondered if dragons existed, and I hope they did.
Explanation:
In "Nights and Dragons", Abigail Prynne reveals the fascination she has had for dragons since she was a kid. She wondered if dragons existed and, by the time she was thirteen, she decided to start researching.
Even though she found out that science would not, in general, support the idea of the existence of dragons, which could be rather discouraging, she never lost hope. She remembered her grandmother's words and chose to think there's more to it than just science.
She was, at last, able to find biologists who claim dragons were not an impossibility for nature. Still, there was no proof, only claims. She ends the text in an undecisive tone, but also a dreamy one. She will keep on wondering; the possibility is still alive.