Answer :
Dr. Jones is using a cross-sectional research design to study IQ changes with age by comparing different age groups at one point in time. So, the correct option is C.
In the scenario described, Dr. Jones is employing a cross-sectional research design to study IQ changes with age. This design involves giving an IQ test to different age groups at one point in time to compare the average IQ of each group. Specifically, Group 1 consists of 10-year-olds, Group 2 of 15-year-olds, and Group 3 of 25-year-olds. This methodology allows Dr. Jones to take a "cross-section" of the population to explore potential differences in IQ across different ages.
However, while this design can reveal age differences, it does not track changes over time in the same individuals, as a longitudinal study would. It also does not combine elements of both cross-sectional and longitudinal designs in the way a cross-sequential or sequential design would. Lastly, Dr. Jones's study does not manipulate any variables or conditions, which distinguishes it from an experimental design, and focuses on observation rather than intervention, which characterizes an observational design. So, the correct option is C.
