Answer:
a. Cross sectional
Explanation:
Longitudinal research is a method in with data is recollected repeatedly over a period of time from the same subjects. Archival method is gathering information from archival records. Cross-sectional studies analyze data from a population at a specific period of time, this population can be integrated by people with different characteristics (in this case, age) but with another characteristics in common, for example, ethnicity, gender, etc)
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