Xiao is using a 'Cross-Sectional' Study to look for developmental differences in each individual.
Explanation:
Cross-sectional study is used to investigate a “variable” which are the “developmental differences” and “number of meals eaten daily” here, in different groups i.e., the mentioned age groups who are related to other characteristics. This research includes a group of people who differ in numbers or variable of interest but there are some characteristics like educational background, socioeconomic status and ethnicity, are shared among them equally.
