Final answer:
The author's purpose is to inform the public about the streetcar plan, considering the informational context about the history and impact of public rail transportation in cities.
Step-by-step explanation:
The author's main purpose in the selection about the streetcar plan is most likely D. to inform the public about the streetcar plan.
The provided references discuss the history and impact of public rail transportation on city development, such as the growth of streetcar suburbs and the evolution of public bus services into subsidized, regulated monopolies. These topics are informative in nature and aim to enhance the reader's understanding of urban transportation systems and their historical progression.