The correct answer is a longitudinal study.
A longitudinal study refers to a research study, where a group of participants are monitored or studied over the span of a couple of years. Longitudinal studies are useful when investigating the development of individuals during their lifespan. In this instance, Dr. Davis is using a longitudinal study to determine how puberty affects adjustment by studying a group of 100 junior high school students over the course of three years, and conducting surveys to measure their adjustment over the years.