The transitional phrase that would be most appropriate to add to the beginning of the sentence is: For this reason,
The correct answer is option D.
The original sentence is: "The new policy reinforced the idea that the United States was a 'good neighbor' to Latin American countries."
Now, let's consider each transitional phrase:
A. That is,
"That is," is typically used to clarify or restate something. In this case, it doesn't flow well because there is no preceding statement that requires clarification.
B. In addition,
"In addition," is used to introduce additional information. While it could work, it may not be the best fit because the sentence seems to be emphasizing the reinforcement of the idea rather than adding new information.
C. In any event,
"In any event," is used to introduce a different perspective or possibility. It doesn't quite fit the context of the sentence, as it doesn't suggest a change in perspective or a contrasting idea.
D. For this reason,
"For this reason," is a strong transitional phrase that indicates a cause-and-effect relationship. It connects the new policy to the reinforcement of the idea that the United States is a "good neighbor" to Latin American countries, making it the most fitting choice in this context.
Therefore, the revised sentence would be: "For this reason, the new policy reinforced the idea that the United States was a 'good neighbor' to Latin American countries." This choice helps maintain the logical flow and coherence of the passage.