Answer: A) Humans were too proud to think that life could exist elsewhere.
Explanation: In the given paragraph we can see the description of how in the nineteenth century, no one wrote about the possibility of the development of intelligent life in other planets. From the given options, the statement that represents the central idea of the paragraph, is the corresponding to option A: Humans were too proud to think that life could exist elsewhere, we can see that in the phrase "Yet so vain is man, and so blinded by his vanity, that no writer, up to the very end of the nineteenth century, expressed any idea that intelligent life might have developed there far, or indeed at all, beyond its earthly level."