In paragraph 1, the use of the words pathfinders and pioneers emphasizes the idea that space is

a-dangerous

b-faraway

c-largely unknown

d-presents a difficult challenge

Correct answer is C. Largely unknown.

Explanation:

Pioneers are people who are the first or among the first, to enter a place, a field of inquiry or progress.

Pathfinders are people who go ahead of others into uncharted territory to discover the way or find new routes.

In the context of the paragraph, "It is an era which calls for action and for the best efforts of all those who would test the unknown and uncertain in every phase of human endeavor. it is time for pathfinders and pioneers."

So the answer is C.