The three statements which best describe the attitude of humans on Earth as they are described in paragraph one of the excerpt are the following ones:
Blank 1:
A. They think they know everything.
"With infinite complacency, men went to and from over this globe about their little affairs, serene in their assurance of their empire over matter."
The author states men busied themselves with their little affairs with complacency, that is, self-satisfied and unaware of any potential danger coming from an unknown place. Plus, it is said that men used to believe that their empire was the only and most important one.
Blank 2:
H. They are unaware of other beings.
"No one gave a thought to the older worlds of space as sources of human danger or thought of them only to dismiss the idea of life upon them as impossible or improbable."
Humans did not take the possibility of life on other planets seriously, since they did not view it as something plausible.
Blank 3:
K They often think about the benefits of exploration.
"At most terrestrial men fancied there might be other men upon Mars, perhaps inferior to themselves and ready to welcome a missionary enterprise."
Even though men did not believe there could be life on other planets, they still supposed (not seriously, though) that the imaginary beings who inhabited Mars would be ready to welcome a missionary enterprise. In other words, men kept the benefits of exploration in mind even when it came to things whose very existence was not entirely valid in their opinion.