Answer: The best question that will expand the research topic is: "What are the state's greatest successes and challenges in improving air quality?"
Explanation: This question will provide Phil with information from multiple perspectives about the state's efforts to control air pollution. By exploring both the successes and challenges faced by the state in improving air quality, Phil will be able to present a more complex and nuanced evaluation of the state's efforts. This will help him provide a more complete picture to his readers, who will be better able to understand the complexities of this issue.