Answer:
correct answer is cross-sectional
Step-by-step explanation:
when a team researcher testy on different age group like 3 months old and 6 months old and 9 months old
so the type of design use is cross-sectional
because cross-sectional is an observational study
and it analysis data from a different age of the population at a specific point in time so it is a cross-sectional method
so correct answer is cross-sectional