It, in fact, proved the feasibility of a transcontinental road and demonstrated that such a line could be built and operated continuously the year around… Based on the context, what does the word "feasibility" mean?
a) Achievability
b) Inevitability
c) Reliability
d) Validity

Based on the context, the word "feasibility" means a) achievability.

Explanation:

The word "feasibility" means viability, the possibility of something being achieved or done. Even if we did not know its meaning, we could have inferred it from the context. The passage says the Pony Express proved the feasibility of a transcontinental road. It demonstrated that such a line - the transcontinental road - could be built and operated. If it can be built and operated, that means the road is achievable, viable, possible.

I've found the whole passage online. That's how I know it speaks of the Pony Express:

The Pony Express marked the highest development in overland travel prior to the coming of the Pacific railroad, which it preceded nine years. It, in fact, proved the feasibility of a transcontinental road and demonstrated that such a line could be built and operated continuously the year around—a feat that had always been regarded as impossible.

the answer is academic paper

and the second should be college professor