Final answer:
The best method to obtain relevant information about the developmental trajectory of a child with Down syndrome transitioning to the school system is through a case study.
Step-by-step explanation:
The BEST method to obtain relevant information about the developmental trajectory of a child with Down syndrome transitioning from early intervention to the school system is through a case study. In a case study, developmental psychologists collect a great deal of information from one individual over time, allowing for a detailed understanding of the child's physical and psychological changes. While there may be researcher bias and difficulty in generalizing conclusions, this approach provides valuable insights into the specific case.