Step 1: Understanding the Concept:
Developmental psychology investigates physical, cognitive, emotional, and social changes across the human lifespan.
Step 2: Detailed Explanation:
The developmental method (comprising longitudinal and cross-sectional studies) systematically traces sequential growth stages, motor milestones, and cognitive behavioral transitions over time in growing children, making it the most comprehensive scientific approach.
Step 3: Final Answer:
Therefore, the best method to study child growth and development is the developmental method.