The most relevant source for an essay with the research question "Why should participating in sports be a requirement for middle schoolers?" would be option C. An article that describes how students who play sports excel at teamwork and problem-solving.
This source would be the most relevant because it directly addresses the benefits of participating in sports for middle schoolers, which is the main focus of the research question. The article provides specific evidence about how playing sports can help students develop important skills such as teamwork and problem-solving, which can be valuable in many areas of their lives.
Option A, a survey about parents' opinions on the demands placed on middle schoolers, is not directly relevant to the research question as it does not address the benefits of participating in sports.
Option B, a website that lists various sports leagues, is not as relevant as option C as it provides information about sports leagues for all ages rather than specifically addressing the benefits of middle schoolers participating in sports.
Option D, a book about volunteering, is not relevant to the research question as it does not address the benefits of sports participation.