The primary purpose of a study investigating the differences between expert and novice performers is to understand the processes and strategies that experts use. Therefore, C is the correct answer. Option A focuses on genetic factors, which is too narrow and not the primary purpose of such studies. Option B is about comparing natural talent, which is not the main focus of these studies. Option D is about the impact of coaching, which is a factor but not the primary purpose of the research. The correct answer is C.