Answer:
A. A panel that consists of households that provide purchasing information at specified intervals over an extended period
Step-by-step explanation:
Longitudinal design in research is a method that involves repeated examination of the same variables over a short or long term to see if there is any changes that occur.
A fixed sample is measured repeatedly to gain information.
A panel that consists of households that provide purchasing information at specified intervals over an extended period, is an example of longitudinal design.
The fixed sample is the panel of households, and they repeatedly provide purchasing information.
So the same sample is measured continuously over a period of time