Answer:
Cross sectional research design
Step-by-step explanation:
Mark is a graduate student who is studying identity formation. He interviews a group of 5-year-olds, a group of 10-year-olds, and a group of 15-year-olds and asks them what they plan to be when they finish school. Mark is using a cross-sectional research design. Mark is studying identity formation on a cross-section of boys. In a cross-sectional design, the researcher is interested in getting data on a cross section of subjects at the same time.