All of the proposed research questions could potentially provide valuable insights into the topic of jazz in the 1930s, but if you're looking for the most effective question in the context of examining the broader impacts of jazz during that decade, "How has music changed over time as a result of jazz in the 1930s?" stands out.
This question allows for an exploration of the influence of 1930s jazz on later musical styles, cultural shifts, and societal impacts. It encourages a comprehensive analysis of how 1930s jazz shaped future genres, musicians, and cultural movements, thereby aligning well with the topic's emphasis on "impacts."