Answer:
Longitudinal
Explanation:
In research, a longitudinal research design is a type of design that involves looking at the variables for a long period of time. During this time, observations are made and repeated during the time that the experiment takes place. This way, a longitudinal research design makes comparisons over time.
In this example, the British Cohort Study followed 11,000 britons over a period of two decades. This sample was tracked during these two decades. Therefore, observations on the sample were made during all this time and this exemplifies the use of a longitudinal research design.