For an essay about how a specific law was passed, the best text structure to use would be chronological order.
This structure allows you to present the sequence of events leading to the law's passage, highlighting key decisions made by lawmakers, important debates, and any significant moments that influenced the process. By organizing the essay chronologically, you can effectively illustrate how the circumstances surrounding the law evolved over time, the rationale behind various choices, and the ultimate outcome.
Additionally, you might incorporate elements of cause and effect within the chronological structure to explain how certain decisions affected the legislative process. This approach would provide a clear and logical flow, making it easier for the audience to understand the complexities involved in passing the law.