To Find Out How Children's Drawing Skills Improve with Age

To find out how children's drawing skills improve with age, a researcher asks 50 four-year-olds to "Draw the best picture of a person that you can." In each of the next six years (until the children are ten years old) , the researcher asks them once again to draw a picture of a person. The researcher then compares quality and detail of the pictures drawn at various ages. Which one of the following research designs does this study best reflect?

A) experimental design
B) correlational design
C) cross-sectional design
D) longitudinal design