Answer:
b.Cross-sectional
Step-by-step explanation:
Cross-sectional research design: In psychological research, the term "cross-sectional research design" is described as one of the types of observational study or research design. However, an experimenter or researcher, while conducting cross-sectional research tends to measure or analyse the outcome and related exposures in the participants involved simultaneously i.e, the research is being conducted at a specific period in time.
In the question above, Xiao is using a cross-sectional research design.