Final answer:
A case study is the most useful technique for determining why a child is afraid to go to school.
Step-by-step explanation:
The most appropriate technique to determine why a child is afraid to go to school is a case study. A case study involves studying a few individuals in depth over an extended period of time, which allows for a detailed understanding of the specific situation. By conducting interviews and observations, researchers can uncover the underlying reasons for the child's fear and gain insights into potential solutions.