Finding the main idea
The village had waited eleven years for the road. When it came, it came in a single season, and by the following summer three families had opened shops, the school's attendance had doubled, and the nearest hospital was ninety minutes away instead of a day. What nobody had predicted was the water. The tankers that had once turned back at the riverbed now reached the settlement twice a week, and the wells, no longer drawn on daily, began to hold.
Option by option
AThe road took eleven years to build.
True and stated, but it is the passage's opening detail, not its point. A main-idea option that repeats the first sentence is almost always wrong.
BA single piece of infrastructure produced effects far beyond the one it was built for.
The passage lists four consequences and singles out the unpredicted one. That structure is the argument.
CThe village's wells were failing before the road arrived.
Implied, but it is a supporting fact inside the final sentence, not what the paragraph is about.
DRural schools improve when transport improves.
Too narrow. The school is one of four examples, and an option naming only one example cannot be the main idea.
Carry this forward
A main-idea option that is true is not thereby right. Test each one by asking whether the rest of the paragraph exists to support it — the main idea is the sentence every other sentence is serving.