The most effective research question depends on the purpose of the research and the specific goals of the researcher. However, in terms of research effectiveness, option B, "How many students in the United States play an instrument?" is the most effective research question because it is specific, measurable, and can be answered using quantitative research methods. The question is clear and focused, and the answer can provide valuable data to inform decision-making and planning related to music education programs. Options A, C, and D are more broad or qualitative in nature and may not provide clear, actionable data.