Final answer:
The text structure that would achieve Seth's purpose is problem-solution.
Step-by-step explanation:
The text structure that would achieve Seth's purpose of writing an article about the reasons for the school board's policy and its impact on students would be problem-solution. A problem-solution text structure presents a problem or issue and then offers a solution or proposed resolutions to that problem. In this case, Seth could discuss the problem of physical inactivity among ninth-grade students and the potential negative effects it can have on their health and well-being. He can then propose the physical education class as a solution to address this problem.