Question

read the passage.
american transit blues
compared to europeans, americans are relatively infrequent users of public transit. in a recent report on transit systems in various u.s. cities, researchers from the university of minnesotas center for transportation studies attempted to address this issue. although underuse of public transportation is a complex problem with many contributing factors, the reports authors noted one significant factor: when cities limit how tall their buildings can be, they tend to end up with more of what urban planners call sprawl. sprawl is the uncontrolled expansion of a citys population away from an urban center into suburban areas. sprawl makes public transit impractical for many residents because they must travel long distances to and from many different points. the authors recommend that cities pass laws to encourage people and businesses to cluster together in centralized areas. the idea is that the resulting high-density areas will be easier to serve with public transit.
what is the main, or central, idea of the passage?
urban sprawl must be controlled in order to provide usable public transportation.
encouraging cities to become denser would likely result in increased public transit use in america.
american residents of urban areas tend to use public transportation less frequently than europeans do.

Explanation:

The passage starts by noting Americans use public transit less than Europeans, identifies urban sprawl (caused by building height limits) as a key barrier to practical public transit, and recommends clustering people/businesses in dense central areas to make transit more viable, which would boost usage. The first option focuses only on controlling sprawl, not the full link to increasing transit use via density. The third option is just an introductory detail, not the central idea. The second option captures the core argument: denser cities would likely increase U.S. public transit use.

Answer:

Encouraging cities to become denser would likely result in increased public transit use in America.