A researcher is designing an experiment to measure if/how development in children

A researcher is designing an experiment to measure if/how development in children is related to regular consumption of a multivitamin. Two hundred children are involved in the study. One hundred will consume a multivitamin daily, and the other one hundred will not. The children will be tracked from ages two years to five years. This research study is an example of:
Sequential design
Longitudinal design
Naturalistic observation
Cross sectional design