Final answer:
The shift from multiple competing city bus companies in the mid-twentieth century to usually one subsidized, regulated monopoly today is primarily due to b.increased government intervention and regulation.
Step-by-step explanation:
In the middle of the twentieth century, there was a significant shift in government policy regarding competition in various industries, including transportation. The change from multiple competing city bus companies to typically a single, subsidized, regulated monopoly can be most directly attributed to increased government intervention and regulation. This change was influenced by a broader historical context where the United States government took action against predatory practices by large monopolies. Legislation beginning in the late 1800s and persisting through much of the twentieth century sought to break up monopolies and regulate industries to ensure fair competition and protect consumers.
Similar examples of government intervention include the regulation of the phone industry, where only AT&T was allowed to provide certain services for an extended period, or the airline industry, which had fare and route limitations imposed by the federal government. These interventions indicate a trend toward government control in sectors deemed essential to the public interest, like transportation. Thus, option b, increased government intervention and regulation, is likely the cause of the consolidation of city bus companies into regulated monopolies.